EXHIBIT 99.1

                CERTIFICATION 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 September 30, 2002 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:  November 13, 2002        /s/ John R. Bertucci
                                 -----------------------------------------------
                                 John R. Bertucci
                                 Chairman, Chief Executive Officer and President