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| <?xml version="1.0" encoding="windows-1252"?> |
| <ownershipDocument> |
| <schemaVersion> X0306 </schemaVersion> |
| <documentType> 4 </documentType> |
| <periodOfReport> 2020-01-27 </periodOfReport> |
| <issuer> |
| | <issuerCik> 0001049502 </issuerCik> |
| | <issuerName> MKS INSTRUMENTS INC </issuerName> |
| | <issuerTradingSymbol> MKSI </issuerTradingSymbol> |
| | </issuer> |
| <reportingOwner> |
| | <reportingOwnerId> |
| | | <rptOwnerCik> 0001775435 </rptOwnerCik> |
| | | <rptOwnerName> Burke Kathleen Flaherty </rptOwnerName> |
| | | </reportingOwnerId> |
| | <reportingOwnerAddress> |
| | | <rptOwnerStreet1> C/O MKS INSTRUMENTS, INC. </rptOwnerStreet1> |
| | | <rptOwnerStreet2> 2 TECH DRIVE, SUITE 201 </rptOwnerStreet2> |
| | | <rptOwnerCity> ANDOVER </rptOwnerCity> |
| | | <rptOwnerState> MA </rptOwnerState> |
| | | <rptOwnerZipCode> 01810 </rptOwnerZipCode> |
| | | <rptOwnerStateDescription></rptOwnerStateDescription> |
| | | </reportingOwnerAddress> |
| | <reportingOwnerRelationship> |
| | | <isOfficer> 1 </isOfficer> |
| | | <officerTitle> Sr. VP, GC & Secretary </officerTitle> |
| | | </reportingOwnerRelationship> |
| | </reportingOwner> |
| <derivativeTable> |
| | <derivativeTransaction> |
| | | <securityTitle> |
| <value> Restricted Stock Unit </value> |
| </securityTitle> |
| | | <conversionOrExercisePrice> |
| <footnoteId id="F1"/> |
| </conversionOrExercisePrice> |
| | | <transactionDate> |
| <value> 2020-01-27 </value> |
| </transactionDate> |
| | | <transactionCoding> |
| <transactionFormType> 4 </transactionFormType> |
| <transactionCode> A </transactionCode> |
| <equitySwapInvolved> 0 </equitySwapInvolved> |
| </transactionCoding> |
| | | <transactionTimeliness></transactionTimeliness> |
| | | <transactionAmounts> |
| <transactionShares> |
| <value> 3872.491 </value> |
| </transactionShares> |
| <transactionPricePerShare> |
| <footnoteId id="F1"/> |
| </transactionPricePerShare> |
| <transactionAcquiredDisposedCode> |
| <value> A </value> |
| </transactionAcquiredDisposedCode> |
| </transactionAmounts> |
| | | <exerciseDate> |
| <footnoteId id="F2"/> |
| </exerciseDate> |
| | | <expirationDate> |
| <footnoteId id="F2"/> |
| </expirationDate> |
| | | <underlyingSecurity> |
| <underlyingSecurityTitle> |
| <value> Common Stock </value> |
| </underlyingSecurityTitle> |
| <underlyingSecurityShares> |
| <value> 3872.491 </value> |
| </underlyingSecurityShares> |
| </underlyingSecurity> |
| | | <postTransactionAmounts> |
| <sharesOwnedFollowingTransaction> |
| <value> 20053.79 </value> |
| </sharesOwnedFollowingTransaction> |
| </postTransactionAmounts> |
| | | <ownershipNature> |
| <directOrIndirectOwnership> |
| <value> D </value> |
| </directOrIndirectOwnership> |
| </ownershipNature> |
| | | </derivativeTransaction> |
| | </derivativeTable> |
| <footnotes> |
| | <footnote id="F1"> Each restricted stock unit represents the contingent right to receive one share of common stock of MKS Instruments, Inc. </footnote> |
| | <footnote id="F2"> These RSUs were subject to performance criteria, achievement of which was determined on January 27, 2020 and thereafter vest in three equal installments beginning on February 15, 2020, provided that if, in any vesting year, February 15th is not a business day, such vesting shall occur on the next business day. </footnote> |
| | </footnotes> |
| <ownerSignature> |
| | <signatureName> /s/ M. Kathryn Rickards, attorney-in-fact </signatureName> |
| | <signatureDate> 2020-01-28 </signatureDate> |
| | </ownerSignature> |
| </ownershipDocument> |
