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  As Of               Filer                 Filing     As/For/On Docs:Pgs              Issuer               Agent

 5/27/05  Exelon Corp                       S-4/A                  7:343                                    Merrill Corp/New/- FA

Document/Exhibit                   Description                      Pages   Size 

 1: S-4/A       Pre-Effective Amendment to Registration of          HTML  2,496K 
                          Securities Issued in a                                 
                          Business-Combination Transaction                       
 2: EX-5        Opinion re: Legality                                   2     14K 
 3: EX-8.1      Opinion re: Tax Matters                                3     15K 
 4: EX-8.2      Opinion re: Tax Matters                                3     16K 
 5: EX-23.1     Consent of Experts or Counsel                          1      7K 
 6: EX-23.2     Consent of Experts or Counsel                          1      8K 
 7: EX-99.1     Miscellaneous Exhibit                                  2     13K 

As filed with the Securities and Exchange Commission on May 27, 2005

Registration No. 333-122704

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

Amendment No. 3
to
Form S-4
REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

Exelon Corporation
(Exact name of registrant as specified in its charter)

10 South Dearborn Street
37th Floor
Chicago, Illinois 60680-5379
(312) 394-7398
(Address, including zip code, and telephone number, including area code, of registrant's principal executive offices)

Randall E. Mehrberg, Esq.
Exelon Corporation
10 South Dearborn Street
37th Floor
Chicago, Illinois 60680-5379
(312) 394-7398
(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies To:

        Approximate date of commencement of proposed sale of the securities to the public:    As soon as practicable after this registration statement becomes effective and after the conditions to the completion of the proposed transaction described in the joint proxy statement/prospectus have been satisfied or waived.

        If the securities being registered on this form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box.    o

        If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

        If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

        The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the registration statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

The Information in this preliminary joint proxy statement/prospectus is not complete and may be changed. These securities may not be sold until the registration statement filed with the Securities and Exchange Commission is effective. This preliminary joint proxy statement/prospectus is not an offer to sell and it is not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

PRELIMINARY COPY—SUBJECT TO COMPLETION, DATED MAY 27, 2005

MERGER PROPOSAL—YOUR VOTE IS IMPORTANT

        On behalf of the boards of directors and management of both Exelon Corporation and Public Service Enterprise Group Incorporated, we are pleased to deliver our joint proxy statement/prospectus for the merger involving Exelon and PSEG. We believe this merger will create a strong combined company that will deliver important benefits to our shareholders, to our customers and to the communities we serve.

        If the merger is completed, PSEG shareholders will receive 1.225 shares of Exelon common stock for each share of PSEG common stock held. The exchange ratio is fixed and will not be adjusted to reflect stock price changes prior to the completion of the merger. Based on the closing price of Exelon common stock on the New York Stock Exchange on December 15, 2004, the last full trading day prior to a significant increase in trading volume from the average trading volume of Exelon and PSEG common stock and inquiries from The Wall Street Journal regarding whether Exelon and PSEG were in advanced merger discussions, the exchange ratio represented approximately $53.14 in value for each share of PSEG common stock. Based on the closing price of Exelon common stock on the New York Stock Exchange on December 16, 2004, the last full trading day prior to the publication of news articles reporting that Exelon and PSEG were in advanced merger discussions, the exchange ratio represented approximately $52.19 in value for each share of PSEG common stock. Based on the closing price of Exelon common stock on the New York Stock Exchange on December 17, 2004, the last full trading day prior to our public announcement of the merger, the exchange ratio represented approximately $51.28 in value for each share of PSEG common stock. Based on the closing price of Exelon common stock on the New York Stock Exchange on May 25, 2005, of $46.67, the exchange ratio represented approximately $57.17 in value for each share of PSEG common stock. The value of the consideration to be received by PSEG shareholders will fluctuate with changes in the price of Exelon common stock. We urge you to obtain current market quotations for Exelon and PSEG common stock.

        Exelon shareholders will continue to own their existing Exelon shares. We estimate that Exelon may issue up to approximately 313 million shares of Exelon common stock to PSEG shareholders as contemplated by the merger agreement, which includes approximately 7.2 million shares of Exelon common stock issuable pursuant to PSEG stock options and other equity-based awards to be assumed by Exelon and substituted with options to purchase Exelon common stock and other Exelon equity-based awards. Upon completion of the merger, Exelon's shareholders immediately prior to the merger will own approximately 68% of Exelon's outstanding common stock on a fully diluted basis and former PSEG shareholders will own approximately 32% of Exelon's outstanding common stock on a fully diluted basis. Upon completion of the merger, Exelon will change its name from Exelon Corporation to Exelon Electric & Gas Corporation. Exelon common stock will continue to be listed on the New York Stock Exchange.

        For a discussion of the United States federal income tax consequences of the merger, see "The Merger—Material United States Federal Income Tax Consequences of the Merger" beginning on page 109 of this joint proxy statement/prospectus.

        We urge you to read this joint proxy statement/prospectus, which includes important information about the merger and our annual meetings. In particular, see the section titled "Risk Factors" on pages 21 through 34 of this joint proxy statement/prospectus which contains a description of the risks that you should consider in evaluating the merger.

        Exelon asks that, in addition to the other matters to be considered at the Exelon annual meeting, Exelon shareholders approve the issuance of shares of Exelon common stock as contemplated by the merger agreement. Exelon's annual meeting will be held at the following time and place:

        July 22, 2005
9:30 a.m., local time
PECO Energy Headquarters
2301 Market Street
Philadelphia, Pennsylvania

        Exelon's board of directors has reviewed and considered the terms of the merger and the merger agreement and has unanimously determined that the merger, including the issuance of shares of Exelon common stock as contemplated by the merger agreement, is advisable, fair to and in the best interests of Exelon and its shareholders and unanimously recommends that Exelon shareholders vote FOR the proposal to approve the issuance of shares of Exelon common stock as contemplated by the merger agreement.

        PSEG asks that, in addition to the other matters to be considered at the PSEG annual meeting, PSEG shareholders approve the merger agreement and thereby approve the merger. PSEG's annual meeting will be held at the following time and place:

        July 19, 2005
2:00 p.m., local time
The New Jersey Performing Arts Center
One Center Street
Newark, New Jersey

        PSEG's board of directors has reviewed and considered the terms of the merger and the merger agreement and has unanimously determined that the merger is advisable, fair to and in the best interests of PSEG and its shareholders and unanimously recommends that PSEG shareholders vote FOR the proposal to approve the merger agreement and thereby approve the merger.

        We cannot complete the merger unless the Exelon shareholders approve the issuance of shares of Exelon common stock as contemplated by the merger agreement and the PSEG shareholders approve the merger agreement. Your vote is important.

        Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved the securities to be issued under this joint proxy statement/prospectus or determined if this joint proxy statement/prospectus is accurate or adequate. Any representation to the contrary is a criminal offense.

        This joint proxy statement/prospectus is dated [    •    ], 2005 and is first being mailed to Exelon and PSEG shareholders on or about [    •    ], 2005.

REFERENCES TO ADDITIONAL INFORMATION

        This joint proxy statement/prospectus incorporates important business and financial information about Exelon and PSEG from other documents that are not included in or delivered with this joint proxy statement/prospectus. This information is available to you without charge upon your written or oral request. You can obtain copies of the documents incorporated by reference into this joint proxy statement/prospectus through the Securities and Exchange Commission website at www.sec.gov or by requesting them in writing or by telephone from the appropriate company at the following addresses and telephone numbers:

        If you would like to request documents from PSEG, please do so by July 12, 2005, in order to receive them before the PSEG annual meeting. If you would like to request documents from Exelon, please do so by July 15, 2005, in order to receive them before the Exelon annual meeting.

        See "Where You Can Find More Information" beginning on page 243 of this joint proxy statement/prospectus.

VOTING BY TELEPHONE, BY THE INTERNET OR BY MAIL

        Exelon shareholders of record may submit their proxies:

•

by telephone, by calling the toll-free number (877) 779-8683 in the United States, Canada or Puerto Rico on a touch-tone phone and following the recorded instructions;



•

by accessing the Internet website at www.eproxyvote.com/exc and following the instructions on the website; or



•

by mail, by indicating your voting preference on the proposals on each proxy card you receive, signing and dating each proxy card and returning each proxy card in the prepaid envelope that accompanied that proxy card.

        PSEG shareholders of record may submit their proxies:

•

by telephone, by calling the toll-free number (866) 242-0618 in the United States, Canada or Puerto Rico on a touch-tone phone and following the recorded instructions;



•

by accessing the Internet website at www.proxyvotenow.com/pseg and following the instructions on the website; or



•

by mail, by indicating your voting preference on the proposals on each proxy card you receive, signing and dating each proxy card and returning each proxy card in the prepaid envelope that accompanied that proxy card.

        If you hold your shares through a bank, broker, custodian or other recordholder, please refer to your proxy card or voting instruction form or the information forwarded by your bank, broker, custodian or other recordholder to see which options are available to you.

NOTICE OF ANNUAL MEETING OF SHAREHOLDERS
OF EXELON CORPORATION

TO THE SHAREHOLDERS OF EXELON CORPORATION:

        Exelon will hold the annual meeting of Exelon shareholders on July 22, 2005, at 9:30 a.m., local time, at PECO Energy Headquarters, 2301 Market Street, Philadelphia, Pennsylvania.

        The purpose of the annual meeting is to consider and take action on the following:

        1.     A proposal to approve the issuance of shares of Exelon common stock as contemplated by the Agreement and Plan of Merger, dated as of December 20, 2004, between Exelon Corporation and Public Service Enterprise Group Incorporated. A copy of the merger agreement is attached as Annex A to the joint proxy statement/prospectus accompanying this notice. In the merger, each share of PSEG common stock outstanding immediately prior to completion of the merger will be converted into the right to receive 1.225 shares of Exelon common stock.

        2.     The election of five Class II directors: Edward A. Brennan, Bruce DeMars, Nelson A. Diaz, John W. Rowe and Ronald Rubin, each for a term of three years.

        3.     A proposal to approve an amendment to Exelon's Amended and Restated Articles of Incorporation to increase the number of authorized shares of Exelon common stock from 1,200,000,000 to 2,000,000,000.

        4.     A proposal to ratify PricewaterhouseCoopers LLP as Exelon's independent accountants for the year 2005.

        5.     A proposal to approve the Exelon Corporation 2006 Long-Term Incentive Plan.

        6.     A proposal to approve the Exelon Corporation Employee Stock Purchase Plan for Unincorporated Subsidiaries.

        7.     Any proposal of the Exelon board of directors to adjourn or postpone the annual meeting.

        8.     Any other business that properly comes before the annual meeting and any adjournment or postponement thereof.

        Shareholders of record of Exelon common stock at the close of business on May 2, 2005 will be entitled to vote at the annual meeting and any adjournment or postponement of that meeting. As of the record date, there were 669,137,081 shares of Exelon common stock outstanding. Each share of common stock is entitled to one vote on each matter properly brought before the meeting.

        You are cordially invited to attend the meeting; however, whether or not you expect to attend in person, you can be sure your shares are represented at the meeting by promptly voting and submitting your proxy by phone, by Internet or by completing, signing, dating and returning the enclosed proxy card in the enclosed prepaid envelope.

        If you plan on attending the annual meeting, please bring your admission ticket and a photo ID along with you. The portion of your proxy card marked "Admission Ticket" or voting instruction form will serve as your admission ticket.

        Your board of directors unanimously recommends that you vote for the proposal to approve the issuance of shares of Exelon common stock as contemplated by the merger agreement, which is described in detail in the joint proxy statement/prospectus accompanying this notice, and for proposals two through seven in this notice.

By Order of the Board of Directors

Katherine K. Combs
Vice President, Corporate Secretary
and Deputy General Counsel

YOUR VOTE IS IMPORTANT

        WE URGE YOU TO VOTE YOUR SHARES AS PROMPTLY AS POSSIBLE BY (1) CALLING THE TOLL-FREE NUMBER (877) 779-8683, (2) ACCESSING THE INTERNET WEBSITE AT www.eproxyvote.com/exc OR (3) COMPLETING, SIGNING, DATING AND RETURNING THE ENCLOSED PROXY CARD.

Public Service Enterprise Group Incorporated
80 Park Plaza, P.O. Box 1171
Newark, New Jersey 07101-1171

NOTICE OF ANNUAL MEETING OF SHAREHOLDERS
OF PUBLIC SERVICE ENTERPRISE GROUP INCORPORATED

TO THE SHAREHOLDERS OF PUBLIC SERVICE ENTERPRISE GROUP INCORPORATED:

        PSEG will hold the annual meeting of PSEG shareholders on July 19, 2005, at 2:00 p.m., local time, at The New Jersey Peforming Arts Center, One Center Street, Newark, New Jersey.

        The purpose of the annual meeting is to consider and take action on the following:

        1.     A proposal to approve the Agreement and Plan of Merger, dated as of December 20, 2004, between Exelon Corporation and Public Service Enterprise Group Incorporated. A copy of the merger agreement is attached as Annex A to the joint proxy statement/prospectus accompanying this notice. In the merger, each share of PSEG common stock outstanding immediately prior to completion of the merger will be converted into the right to receive 1.225 shares of Exelon common stock.

        2.     To elect three members of Class III of the PSEG board of directors: Conrad K. Harper, Shirley Ann Jackson and Thomas A. Renyi.

        3.     To consider and act upon the ratification of the appointment of Deloitte & Touche LLP as PSEG's independent auditor for the year 2005.

        4.     To consider and act upon any proposal by the PSEG board of directors to adjourn or postpone the PSEG annual meeting.

        5.     To consider and act upon a shareholder proposal related to executive compensation, if presented at the meeting.

        6.     Any other business that properly comes before the annual meeting and any adjournment or postponement thereof.

        Shareholders of record of PSEG common stock at the close of business on May 27, 2005 will be entitled to vote at the annual meeting and any adjournment or postponement of that meeting. As of the record date, there were 238,731,592 shares of PSEG common stock outstanding. Each share of common stock is entitled to one vote on each matter properly brought before the meeting, except for the proposal relating to the election of directors, on which PSEG shareholders are entitled to cumulative voting.

        You are cordially invited to attend the meeting; however, whether or not you expect to attend in person, you can be sure your shares are represented at the meeting by promptly voting and submitting your proxy by phone, by Internet or by completing, signing, dating and returning the enclosed proxy card in the enclosed prepaid envelope.

        If you plan on attending the annual meeting, please bring your admission ticket and a photo ID along with you. The portion of your proxy card marked "Admission Ticket" or voting instruction form will serve as your admission ticket.

        Your board of directors unanimously recommends that you vote for the proposal to approve the merger agreement and thereby approve the merger, which is described in detail in the joint proxy statement/prospectus accompanying this notice, and for proposals two through four in this notice and against proposal five in this notice.

YOUR VOTE IS IMPORTANT
 


        WE URGE YOU TO VOTE YOUR SHARES AS PROMPTLY AS POSSIBLE BY (1) CALLING THE TOLL-FREE NUMBER (866) 242-0618, (2) ACCESSING THE INTERNET WEBSITE AT www.proxyvotenow.com/pseg OR (3) COMPLETING, SIGNING, DATING AND RETURNING THE ENCLOSED PROXY CARD.

        PLEASE DO NOT SEND YOUR COMMON STOCK CERTIFICATES AT THIS TIME. IF THE MERGER IS COMPLETED, YOU WILL BE SENT INSTRUCTIONS REGARDING THE SURRENDER OF YOUR CERTIFICATES.

   
TABLE OF CONTENTS

i

ii

iii

   
CHAPTER ONE
THE INTRODUCTION

QUESTIONS AND ANSWERS ABOUT THE ANNUAL MEETINGS

Q:

Why am I receiving this document?

A:

We are delivering this document to you because it is serving as both a joint proxy statement of Exelon and PSEG and a prospectus of Exelon. It is a joint proxy statement because it is being used by our boards of directors to solicit proxies of our shareholders. It is a prospectus because Exelon is offering shares of its common stock in exchange for shares of PSEG common stock if the merger is completed.

Q:

What do I need to do now?

A:

After you carefully read this joint proxy statement/prospectus, please respond by submitting your proxy by telephone or the Internet or by completing, signing, dating and returning your signed proxy card(s) in the enclosed prepaid return envelope, as soon as possible, so that your shares may be represented at your annual meeting. In order to assure that your vote is recorded, please vote your proxy as instructed on your proxy card even if you currently plan to attend your annual meeting in person.

Q:

Why is my vote important?

A:

Exelon shareholders are being asked to approve the issuance of shares of Exelon common stock as contemplated by the merger agreement and to take action on the other proposals in the Exelon notice. PSEG shareholders are being asked to approve the merger agreement and thereby approve the merger and to take action on the other proposals in the PSEG notice. If you do not submit your proxy by telephone or the Internet, or return your signed proxy card(s) by mail or vote in person at your annual meeting, it will be more difficult for Exelon and PSEG to obtain the necessary quorum to hold their respective annual meetings.

Q:

How will my proxy be voted?

A:

If you vote by telephone or by the Internet or by completing, signing, dating and returning your signed proxy card(s), your proxy will be voted in accordance with your instructions.

If you are an Exelon shareholder of record and submit your proxy but do not indicate how you want to vote, your shares will be voted FOR the proposal to approve the issuance of shares of Exelon common stock as contemplated by the merger agreement, FOR the proposal to elect the directors named in the director proposal, FOR the proposal to approve the amendment to Exelon's Amended and Restated Articles of Incorporation to increase the number of authorized shares of Exelon common stock, FOR the proposal to ratify PricewaterhouseCoopers LLP as Exelon's independent accountants, FOR the proposal to approve the Exelon Corporation 2006 Long-Term Incentive Plan, FOR the proposal to approve the Exelon Corporation Employee Stock Purchase Plan for Unincorporated Subsidiaries and FOR any proposal by the Exelon board of directors to adjourn or postpone the annual meeting, if necessary.

If you are a PSEG shareholder of record and submit your proxy but do not indicate how you want to vote, your shares will be voted FOR the proposal to approve the merger agreement, FOR the proposal to elect the directors named in the director proposal, FOR the proposal to ratify Deloitte & Touche LLP as PSEG's independent auditor, FOR any proposal by the PSEG board of directors to adjourn or postpone the annual meeting, if necessary, and AGAINST the shareholder proposal.

1

Q:

May I vote in person?

A:

Yes. If you are a shareholder of record of Exelon common stock as of May 2, 2005 or of PSEG common stock as of May 27, 2005, you may attend your annual meeting and vote your shares in person, instead of submitting your proxy by telephone or by the Internet or returning your signed proxy card(s).

Q:

If my shares are held in "street name" by my broker, will my broker vote my shares for me?

A:

If you are an Exelon shareholder whose shares are held in "street name" by your broker, you must provide your broker with instructions on how to vote your shares; otherwise, your broker will not vote your shares on the proposal to approve the issuance of shares of Exelon common stock as contemplated by the merger agreement, the proposal to approve the Exelon Corporation 2006 Long-Term Incentive Plan or the proposal to approve the Exelon Corporation Employee Stock Purchase Plan for Unincorporated Subsidiaries.

If you are a PSEG shareholder whose shares are held in "street name" by your broker, you must provide your broker with instructions on how to vote your shares; otherwise, your broker will not vote your shares on the proposal to approve the merger agreement or the shareholder proposal.

You should be sure to provide your broker with instructions on how to vote your shares. Please check the voting form used by your broker to see if it offers telephone or Internet submission of proxies.

Q:

What if I fail to instruct my broker?

A:

If you fail to instruct your broker to vote your shares and the broker submits an unvoted proxy, the resulting broker "non-vote" will, in the circumstances described under "Chapter Two—Information About the Annual Meetings and Voting," be counted toward a quorum at your annual meeting but will not be counted in determining the number of votes cast with respect to a proposal.

Q:

Who will count the vote?

A:

For the Exelon proposals, representatives of Equiserve Trust Company, N.A. and Exelon's Office of the Corporate Secretary will count the votes and serve as judges of election.

For the PSEG proposals, representatives of Corporate Election Services, Inc. will count the vote and serve as inspectors of election.

Q:

What does it mean if I receive more than one set of materials?

A:

This means you own shares of both Exelon and PSEG or you own shares of Exelon or PSEG that are registered under different names. For example, you may own some shares directly as a shareholder of record and other shares through a broker or you may own shares through more than one broker. In these situations, you will receive multiple sets of proxy materials. It is necessary for you to vote, sign and return all of the proxy cards or follow the instructions for any alternative voting procedure on each of the proxy cards you receive in order to vote all of the shares you own. Each proxy card you receive will come with its own prepaid return envelope; if you vote by mail, make sure you return each proxy card in the return envelope which accompanied that proxy card.

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Q:

Can I revoke my proxy and change my vote?

A:

Yes. You have the right to revoke your proxy at any time prior to the time your shares are voted at your annual meeting. If you are a shareholder of record, your proxy can be revoked in several ways:

•

by entering a new vote by telephone or the Internet;

•

by delivering a written revocation to your company's secretary that is received prior to your annual meeting;

•

by submitting another valid proxy bearing a later date that is received prior to your annual meeting; or

•

by attending your annual meeting and voting your shares in person.

However, if your shares are held in "street name" through a bank, broker, custodian or other recordholder, you must check with your bank, broker, custodian or other recordholder to determine how to revoke your proxy.

Q:

When and where are the annual meetings?

A:

The Exelon annual meeting will take place on July 22, 2005, at 9:30 a.m., local time, at PECO Energy Headquarters, 2301 Market Street, Philadelphia, Pennsylvania.

The PSEG annual meeting will take place on July 19, 2005, at 2:00 p.m., local time, at The New Jersey Performing Arts Center, One Center Street, Newark, New Jersey.

Q:

What must I bring to attend the annual meetings?

A:

Admittance to the annual meetings is limited to shareholders of Exelon or PSEG, as the case may be, who are eligible to vote or their authorized representatives. If you wish to attend your meeting, bring the portion of your proxy which is marked as your "Admission Ticket" or your voting instruction form. Also, be sure to bring a photo ID. Seating is limited.

Q:

Should I send in my stock certificates now?

A:

No. After the merger is completed, Exelon will send PSEG shareholders written instructions for exchanging their PSEG stock certificates for Exelon stock certificates. Exelon shareholders will keep their existing stock certificates.

Q:

Are there risks I, as an Exelon shareholder, should consider in deciding to vote on the issuance of shares of Exelon common stock as contemplated by the merger agreement or, as a PSEG shareholder, should consider in deciding to vote on the approval of the merger agreement?

A:

Yes, in evaluating the issuance of shares of Exelon common stock as contemplated by the merger agreement, or the merger agreement and the merger, you should carefully read this joint proxy statement/prospectus, including the factors discussed in the section titled "Risk Factors" beginning on page 21 of this joint proxy statement/prospectus.

Q:

Who can answer any questions I may have about the annual meetings or the merger?

A:

Exelon shareholders may call Georgeson Shareholder Communications Inc. at (800) 561-4182.

PSEG shareholders may call Morrow & Co., Inc. at (800) 607-0088.

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SUMMARY

        This summary highlights selected information from this joint proxy statement/prospectus related to the merger and may not contain all of the information that is important to you. To understand the merger and for a more complete description of the legal terms of the merger agreement, you should carefully read this entire joint proxy statement/prospectus and the documents to which this joint proxy statement/prospectus refers you. A copy of the merger agreement is attached as Annex A to this joint proxy statement/prospectus and is incorporated by reference into this joint proxy statement/prospectus. See "Where You Can Find More Information" on page 243 of this joint proxy statement/prospectus.

The Companies Involved in the Merger (see page 37)

Exelon Corporation
10 South Dearborn Street—37th Floor
P.O. Box 805379
Chicago, Illinois 60680-5379
(312) 394-7398
Internet address: www.exeloncorp.com

        Exelon Corporation, a registered public utility holding company under the Public Utility Holding Company Act of 1935, as amended, through its subsidiaries, operates in two business segments—Energy Delivery and Generation as of January 1, 2005. Prior to January 1, 2005, Exelon operated in a third business segment, Enterprises. In addition to Exelon's two business segments, Exelon Business Services Company, a subsidiary of Exelon, provides Exelon and its subsidiaries with financial, human resource, legal, information technology, supply management and corporate governance services. Exelon was incorporated in Pennsylvania in February 1999.

Public Service Enterprise Group Incorporated
80 Park Plaza
P.O. Box 1171
Newark, New Jersey 07101-1171
(973) 430-7000
Internet address: www.pseg.com

        Public Service Enterprise Group Incorporated is an exempt public utility holding company under PUHCA. PSEG has three principal direct wholly-owned subsidiaries engaged in the transmission and distribution of electric energy and gas in New Jersey; wholesale supply; and power production and distribution in selected domestic and international markets. In addition, PSEG Services provides management and administrative services to PSEG and its subsidiaries. PSEG was incorporated under the laws of the State of New Jersey in 1985.

The Merger (see page 118)

        Under the terms of the merger, PSEG will merge with and into Exelon with Exelon continuing as the surviving corporation. Upon completion of the merger, Exelon will change its name to Exelon Electric & Gas Corporation.

        The merger agreement is attached as Annex A to this joint proxy statement/prospectus. We urge you to read the merger agreement carefully and fully to understand the rights and obligations of Exelon and PSEG under the merger agreement, as it is the legal document that governs the merger.

PSEG Shareholders Will Receive 1.225 Shares of Exelon Common Stock for Each Share of PSEG Common Stock (see page 118)

        Subject to the terms and conditions of the merger agreement, upon completion of the merger, PSEG shareholders will receive 1.225 shares of Exelon common stock for each share of PSEG common stock they hold. Exelon will not issue fractional shares pursuant to the merger. As a result, the total

4

number of shares of Exelon common stock that each PSEG shareholder would otherwise receive pursuant to the merger will be rounded down to the nearest whole number, and each PSEG shareholder will receive such whole number of shares of Exelon common stock and a cash payment for the remaining fraction of a share of Exelon common stock that such shareholder would otherwise receive, if any, based on the last reported sale price per share of Exelon common stock at the close of business on the closing date of the merger, rounded down to the nearest cent.

        Example: If you own 137 shares of PSEG common stock when the merger is completed, you will be entitled to receive 167 shares of Exelon common stock and a check for the market value of 0.83 shares of Exelon common stock at the close of business on the closing date of the merger.

Each Outstanding Option to Purchase PSEG Common Stock and Each PSEG Equity-Based Award Will be Substituted with an Option to Purchase Shares of Exelon Common Stock or an Exelon Equity-Based Award at the Exchange Ratio and All Restrictions on PSEG Equity-Based Awards Will Be Assigned to Exelon (see page 118)

        Upon completion of the merger, each outstanding option to purchase shares of PSEG common stock will be assumed by Exelon and substituted with an option to purchase shares of Exelon common stock, exercisable on generally the same terms and conditions that applied before the merger. The number of shares of Exelon common stock subject to the substitute Exelon stock option will equal the number of shares of PSEG common stock subject to the PSEG stock option immediately prior to completion of the merger, multiplied by the exchange ratio, rounded down to the nearest whole share. The per share exercise price of each substitute Exelon stock option will equal the exercise price of the PSEG stock option immediately prior to completion of the merger divided by the exchange ratio, rounded up to the nearest whole cent. In addition, upon completion of the merger, Exelon will assume all PSEG equity-based awards and substitute them with equity-based awards with respect to shares of Exelon common stock on generally the same terms and conditions that applied before completion of the merger. The number of shares of Exelon common stock issuable under those awards, and the exercise prices for those awards, will be adjusted to take into account the exchange ratio.

        Upon completion of the merger, all restrictions on PSEG equity-based awards immediately prior to completion of the merger, including all repurchase and forfeiture rights, will be assigned to Exelon, and, subject to any acceleration, lapse or other vesting occurring by operation of the merger, those PSEG equity-based awards will continue to be unvested and subject to the same restrictions which applied immediately prior to completion of the merger.

The Exchange Ratio is Fixed and Will Not Be Adjusted in Response to Changes in Our Stock Prices (see page 118)

        The exchange ratio is fixed in the merger agreement and neither Exelon nor PSEG has the right to terminate the merger agreement based solely on changes in either party's stock price. The market value of the Exelon common stock that PSEG shareholders receive in the merger may fluctuate significantly from its current value.

        The table below shows the closing prices of Exelon and PSEG common stock, which are listed on the New York Stock Exchange under the trading symbols "EXC" and "PEG," respectively, and the pro forma "equivalent stock price" at the close of the regular trading session on:

•

December 15, 2004, the last full trading day prior to a significant increase in trading volume from the average trading volume of Exelon and PSEG common stock and inquiries from The Wall Street Journal regarding whether Exelon and PSEG were in advanced merger discussions;



•

December 16, 2004, the last full trading day prior to the publication of news articles reporting that Exelon and PSEG were in advanced merger discussions;

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•

December 17, 2004, the last full trading day prior to our public announcement of the merger; and



•

May 25, 2005, the most recent trading day for which that information was available prior to the mailing of this joint proxy statement/prospectus.

        The "equivalent stock price" of shares of PSEG common stock represents the closing price per share of Exelon common stock on the New York Stock Exchange as of the applicable date, multiplied by the exchange ratio of 1.225.

        The value of the consideration to be received by PSEG shareholders will fluctuate with changes in the price of Exelon common stock. You are urged to obtain current market quotations for shares of both companies.

Exelon and PSEG Expect to Continue Their Respective Stated Dividend Policies Until Completion of the Merger; Exelon has Agreed, Subject to Specified Limitations, to Increase its Dividend Following Completion of the Merger to Equal PSEG's Dividend on an Exchange Ratio Adjusted Basis

        The merger agreement permits each of us to continue to pay regular dividends to our respective shareholders in accordance with our previously announced dividend policies. Exelon has previously indicated it expects to maintain a dividend payout policy of 50% to 60% of earnings. On April 27, 2005, Exelon declared a second quarter dividend for 2005 of $0.40 per share. On January 18, 2005, PSEG increased its first quarter dividend for 2005 to $0.56 per share from $0.55 per share, for an indicated annual dividend increase of $0.04 per share for the year 2005. On April 19, 2005, PSEG declared a second quarter dividend for 2005 of $0.56 per share. For the year 2006, PSEG will continue to evaluate its dividend payment and consider modest increases.

        We have agreed to coordinate dividend declarations and the related record dates and payment dates so that our shareholders will not receive two dividends, or fail to receive one dividend, for any single calendar quarter. Accordingly, prior to completion of the merger, we may coordinate and amend our record dates and payment dates in order to effect this policy.

        In addition, the merger agreement provides that, subject to applicable law and the fiduciary duties of its board of directors, Exelon will increase its first quarterly dividend paid after completion of the merger to an amount equal, on an exchange ratio adjusted basis, to the dividend PSEG shareholders received in the quarter immediately prior to completion of the merger, up to a maximum of $0.47 per share of Exelon common stock. The lesser of $0.47 and the amount required to equal PSEG's dividend on an exchange ratio adjusted basis is referred to in this joint proxy statement/prospectus as the "threshold amount." If the first quarterly dividend to be paid by Exelon after completion of the merger will be less than the threshold amount, PSEG may make a one time special cash dividend to its shareholders equal to the amount of the difference between the dividend Exelon has informed PSEG it will pay and the threshold amount, on an exchange ratio adjusted basis.

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PSEG Shareholders Generally Will Not Recognize Any Gain or Loss for United States Federal Income Tax Purposes Upon the Exchange of Their Shares of PSEG Common Stock for Shares of Exelon Common Stock in the Merger, Except that Gain or Loss Will Be Recognized on the Receipt of Cash in Lieu of a Fractional Share of Exelon Common Stock (see page 109)

        A holder of shares of PSEG common stock generally will not recognize any gain or loss upon the exchange of the holder's shares of PSEG common stock for shares of Exelon common stock pursuant to the merger, except that gain or loss will be recognized on the receipt of cash in lieu of a fractional share of Exelon common stock. You should read carefully the discussion under the heading "The Merger—Material United States Federal Income Tax Consequences of the Merger" beginning on page 109 of this joint proxy statement/prospectus.

        Holders of shares of PSEG common stock are urged to consult their tax advisors as to the specific tax consequences to them of the merger, including the applicability and effect of United States federal, state, local and foreign income and other tax laws in light of their particular circumstances.

Exelon's Financial Advisors Delivered their Opinions to the Exelon Board of Directors to the Effect that, as of December 20, 2004, the Exchange Ratio in the Merger Was Fair, from a Financial Point of View, to Exelon (see page 66)

        Each of Exelon's financial advisors, J.P. Morgan Securities Inc. and Lehman Brothers Inc., has delivered its opinion dated as of December 20, 2004 to the Exelon board of directors that, as of that date, based upon and subject to the assumptions, qualifications and limitations set forth in their respective opinions, the exchange ratio in the merger was fair, from a financial point of view, to Exelon. The full text of the opinions of JPMorgan and Lehman Brothers are attached as Annex B and Annex C, respectively, to this joint proxy statement/prospectus. Exelon urges its shareholders to read the opinions in their entirety. Each of JPMorgan and Lehman Brothers has provided its opinion for the information and assistance of the Exelon board of directors in connection with its consideration of the merger agreement, the issuance of shares of Exelon common stock as contemplated by the merger agreement and the merger, and the opinions do not constitute a recommendation as to how any holder of Exelon common stock should vote with respect to the issuance of shares of Exelon common stock as contemplated by the merger agreement.

        The opinion of each of JPMorgan and Lehman Brothers will not reflect any developments that may occur or may have occurred after the date of the opinions and prior to completion of the merger.

        Pursuant to an engagement letter dated October 26, 2004, Exelon has agreed to pay JPMorgan a fee of $15 million in consideration for its services as financial advisor, $5 million of which was paid following the public announcement of the execution of the merger agreement, $5 million of which is payable upon approval of the issuance of shares of Exelon common stock as contemplated by the merger agreement by Exelon shareholders and $5 million of which is payable upon completion of the merger. Pursuant to an engagement letter dated November 5, 2004, Exelon has agreed to pay Lehman Brothers a fee of $15 million in consideration for its services as financial advisor, $5 million of which was paid following the public announcement of the execution of the merger agreement, $5 million of which is payable upon approval of the issuance of shares of Exelon common stock as contemplated by the merger agreement by Exelon shareholders and $5 million of which is payable upon completion of the merger.

PSEG's Financial Advisor Delivered its Opinion to the PSEG Board of Directors to the Effect that, as of December 20, 2004, the Exchange Ratio Under the Merger Agreement Was Fair, from a Financial Point of View, to the Holders of PSEG Common Stock (see page 82)

        PSEG's financial advisor, Morgan Stanley & Co. Incorporated, has delivered its opinion dated as of December 20, 2004 to the PSEG board of directors that, as of that date, based upon and subject to the assumptions, qualifications and limitations discussed in its opinion, the exchange ratio under the

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merger agreement was fair, from a financial point of view, to the holders of PSEG common stock. The full text of Morgan Stanley's opinion is attached as Annex D to this joint proxy statement/prospectus. PSEG urges its shareholders to read that opinion in its entirety. Morgan Stanley provided its opinion for the information and assistance of the PSEG board of directors in connection with its consideration of the merger agreement and the merger, and the opinion does not constitute a recommendation as to how any holder of PSEG common stock should vote with respect to the merger agreement.

        The opinion of Morgan Stanley will not reflect any developments that may occur or may have occurred after the date of its opinion and prior to completion of the merger.

        Pursuant to an engagement letter dated November 8, 2004, PSEG has agreed to pay Morgan Stanley a fee of $20 million in consideration for its services as financial advisor, $5 million of which was paid following the public announcement of the execution of the merger agreement, $5 million of which is payable upon PSEG shareholder approval of the merger agreement and $10 million of which is payable upon completion of the merger.

Both Exelon and PSEG Shareholder Approvals Will Be Required to Complete the Merger (see pages 40 and 45)

For Exelon Shareholders:

        Approval of the proposal to issue shares of Exelon common stock as contemplated by the merger agreement requires the affirmative vote of at least a majority of the votes cast by holders of shares of Exelon common stock present in person or by proxy and entitled to vote on the issuance of shares of Exelon common stock as contemplated by the merger agreement as long as a quorum, which is the presence of holders of shares of Exelon common stock outstanding and entitled to cast at least a majority of the votes that all shareholders are entitled to cast on the matter, is present in person or by proxy and the total votes cast on the proposal represents at least a majority of the shares of Exelon common stock entitled to vote. Approval of the proposal to issue shares of Exelon common stock as contemplated by the merger agreement is a condition to completion of the merger.

        Because the holders of Exelon common stock immediately prior to completion of the merger will own a majority of the shares of Exelon common stock outstanding immediately following completion of the merger, a separate vote by the holders of Exelon common stock on the merger agreement or the merger itself is not required under Pennsylvania law.

        On May 2, 2005, which is the record date for determining those Exelon shareholders who are entitled to vote at the Exelon annual meeting, directors and executive officers of Exelon and their affiliates beneficially owned and had the right to vote 4,955,493 shares of Exelon common stock, representing less than 1% of the shares of Exelon common stock outstanding on the record date. To Exelon's knowledge, directors and executive officers of Exelon and their affiliates intend to vote their shares of common stock in favor of the proposal to issue shares of Exelon common stock as contemplated by the merger agreement, although none of them has entered into any agreement requiring them to do so.

For PSEG Shareholders:

        Approval of the merger agreement requires the affirmative vote of at least a majority of the votes cast by holders of shares of PSEG common stock present in person or by proxy and entitled to vote on the approval of the merger agreement as long as a quorum, which is a majority of the shares of PSEG common stock outstanding, is present in person or by proxy.

        On May 27, 2005, which is the record date for determining those PSEG shareholders who are entitled to vote at the PSEG annual meeting, directors and executive officers of PSEG and their affiliates beneficially owned and had the right to vote 4,951,871 shares of PSEG common stock, representing approximately 1.8% of the outstanding shares of PSEG common stock outstanding on the

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record date. To PSEG's knowledge, directors and executive officers of PSEG and their affiliates intend to vote their shares of common stock in favor of the approval of the merger agreement, although none of them has entered into any agreement requiring them to do so.

Recommendations of Exelon and PSEG to Shareholders (see pages 57 and 62)

To Exelon Shareholders:

        The Exelon board of directors has reviewed and considered the terms of the merger and the merger agreement and has unanimously determined that the merger, including the issuance of shares of Exelon common stock as contemplated by the merger agreement, is advisable, fair to and in the best interests of Exelon and its shareholders and unanimously recommends that Exelon shareholders vote FOR the proposal to approve the issuance of shares of Exelon common stock as contemplated by the merger agreement.

To PSEG Shareholders:

        The PSEG board of directors has reviewed and considered the terms of the merger and the merger agreement and has unanimously determined that the merger i