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 Annual Report on Form 10-K of MKS Instruments, Inc.
(the "Company") for the period ended December 31, 2003 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), the
undersigned, John R. Bertucci, Chairman, Chief Executive Officer and President
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:  March 8, 2004        /s/ John R. Bertucci
                             -----------------------------------------------
                             John R. Bertucci
                             Chairman, Chief Executive Officer and President