EXHIBIT 32.1

                    CERTIFICATION OF CHIEF EXECUTIVE OFFICER

                       PURSUANT TO 18 U.S.C. SECTION 1350,

                             AS ADOPTED PURSUANT TO

                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

       In connection with the Quarterly Report on Form 10-Q of MKS Instruments,
Inc. (the "Company") for the period ended March 31, 2004 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), the
undersigned, John R. Bertucci, Chairman and Chief Executive Officer of the
Company, hereby certifies, pursuant to 18 U.S.C. Section 1350, that:

       (1)    the Report fully complies with the requirements of Section 13(a)
              or 15(d) of the Securities Exchange Act of 1934; and

       (2)    the information contained in the Report fairly presents, in all
              material respects, the financial condition and results of
              operations of the Company.

Dated:  May 5, 2004                             /s/ John R. Bertucci
                                            ------------------------------------
                                            John R. Bertucci
                                            Chairman and Chief Executive Officer