content: <?xml version="1.0" encoding="UTF-8"?><edgarSubmission xmlns="http://www.sec.gov/edgar/ownership" xmlns:com="http://www.sec.gov/edgar/common">
    <headerData>
      <submissionType>144</submissionType>
      <filerInfo>
        <filer>
          <filerCredentials>
            <cik>0001501968</cik>
            <ccc>XXXXXXXX</ccc>
          </filerCredentials>
        </filer>
        <liveTestFlag>LIVE</liveTestFlag>
  
  
  
      </filerInfo>
    </headerData>
    <formData>
      <issuerInfo>
        <issuerCik>0001049502</issuerCik>
        <issuerName>MKS INSTRUMENTS INC</issuerName>
        <secFileNumber>000-23621</secFileNumber>
        <issuerAddress>
          <com:street1>2 TECH DRIVE</com:street1>
          <com:street2>SUITE 201</com:street2>
          <com:city>ANDOVER</com:city>
          <com:stateOrCountry>MA</com:stateOrCountry>
          <com:zipCode>01810</com:zipCode>
        </issuerAddress>
        <issuerContactPhone>978-645-5500</issuerContactPhone>
        <nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>Moloney Jacqueline F</nameOfPersonForWhoseAccountTheSecuritiesAreToBeSold>
        <relationshipsToIssuer>
          <relationshipToIssuer>Director</relationshipToIssuer>
        </relationshipsToIssuer>
      </issuerInfo>
      <securitiesInformation>
        <securitiesClassTitle>Common</securitiesClassTitle>
        <brokerOrMarketmakerDetails>
          <name>Fidelity Brokerage Services LLC</name>
          <address>
            <com:street1>900 Salem Street</com:street1>
            <com:city>Smithfield</com:city>
            <com:stateOrCountry>RI</com:stateOrCountry>
            <com:zipCode>02917</com:zipCode>
          </address>
        </brokerOrMarketmakerDetails>
        <noOfUnitsSold>1918</noOfUnitsSold>
        <aggregateMarketValue>237932.66</aggregateMarketValue>
        <noOfUnitsOutstanding>66878495</noOfUnitsOutstanding>
        <approxSaleDate>02/16/2024</approxSaleDate>
        <securitiesExchangeName>NASDAQ</securitiesExchangeName>
      </securitiesInformation>
      <securitiesToBeSold>
        <securitiesClassTitle>Common</securitiesClassTitle>
        <acquiredDate>05/08/2018</acquiredDate>
        <natureOfAcquisitionTransaction>Restricted Stock Vesting</natureOfAcquisitionTransaction>
        <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
        <isGiftTransaction>N</isGiftTransaction>
        <amountOfSecuritiesAcquired>932</amountOfSecuritiesAcquired>
        <paymentDate>05/08/2018</paymentDate>
        <natureOfPayment>Compensation</natureOfPayment>
      </securitiesToBeSold>
      <securitiesToBeSold>
        <securitiesClassTitle>Common</securitiesClassTitle>
        <acquiredDate>05/10/2021</acquiredDate>
        <natureOfAcquisitionTransaction>Restricted Stock Vesting</natureOfAcquisitionTransaction>
        <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
        <isGiftTransaction>N</isGiftTransaction>
        <amountOfSecuritiesAcquired>839</amountOfSecuritiesAcquired>
        <paymentDate>05/10/2021</paymentDate>
        <natureOfPayment>Compensation</natureOfPayment>
      </securitiesToBeSold>
      <securitiesToBeSold>
        <securitiesClassTitle>Common</securitiesClassTitle>
        <acquiredDate>05/09/2022</acquiredDate>
        <natureOfAcquisitionTransaction>Restricted Stock Vesting</natureOfAcquisitionTransaction>
        <nameOfPersonfromWhomAcquired>Issuer</nameOfPersonfromWhomAcquired>
        <isGiftTransaction>N</isGiftTransaction>
        <amountOfSecuritiesAcquired>147</amountOfSecuritiesAcquired>
        <paymentDate>05/09/2022</paymentDate>
        <natureOfPayment>Compensation</natureOfPayment>
      </securitiesToBeSold>
      <nothingToReportFlagOnSecuritiesSoldInPast3Months>N</nothingToReportFlagOnSecuritiesSoldInPast3Months>
      <securitiesSoldInPast3Months>
        <sellerDetails>
          <name>Jacqueline F. Moloney</name>
          <address>
            <com:street1>2 Tech Drive</com:street1>
            <com:street2>Suite 201</com:street2>
            <com:city>Andover</com:city>
            <com:stateOrCountry>MA</com:stateOrCountry>
            <com:zipCode>01810</com:zipCode>
          </address>
        </sellerDetails>
        <securitiesClassTitle>Common</securitiesClassTitle>
        <saleDate>12/01/2023</saleDate>
        <amountOfSecuritiesSold>250</amountOfSecuritiesSold>
        <grossProceeds>20557.50</grossProceeds>
      </securitiesSoldInPast3Months>
      <noticeSignature>
        <noticeDate>02/16/2024</noticeDate>
        <signature>/s/ Courtney Broxterman, as a duly authorized representative of Fidelity Brokerage Services LLC, as attorney-in-fact for Jacqueline F. Moloney</signature>
      </noticeSignature>
    </formData>
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text: 144
  0001501968
  XXXXXXXX
  LIVE
  0001049502
  MKS INSTRUMENTS INC
  000-23621
  2 TECH DRIVE
  SUITE 201
  ANDOVER
  MA
  01810
  978-645-5500
  Moloney Jacqueline F
  Director
  Common
  Fidelity Brokerage Services LLC
  900 Salem Street
  Smithfield
  RI
  02917
  1918
  237932.66
  66878495
  02/16/2024
  NASDAQ
  Common
  05/08/2018
  Restricted Stock Vesting
  Issuer
  N
  932
  05/08/2018
  Compensation
  Common
  05/10/2021
  Restricted Stock Vesting
  Issuer
  N
  839
  05/10/2021
  Compensation
  Common
  05/09/2022
  Restricted Stock Vesting
  Issuer
  N
  147
  05/09/2022
  Compensation
  N
  Jacqueline F. Moloney
  2 Tech Drive
  Suite 201
  Andover
  MA
  01810
  Common
  12/01/2023
  250
  20557.50
  02/16/2024
  /s/ Courtney Broxterman, as a duly authorized representative of Fidelity Brokerage Services LLC, as attorney-in-fact for Jacqueline F. Moloney