(MKS LOGO)

                                                           FOR IMMEDIATE RELEASE

                                                                        CONTACT:
                                                                  Ronald Weigner
                                        Vice President & Chief Financial Officer
                                                                    978.975.2350

          MKS INSTRUMENTS ELECTS LEO BERLINGHIERI AS PRESIDENT AND COO

      Wilmington, Mass., April 26, 2004, MKS Instruments, Inc. (NASDAQ: MKSI), a
leading worldwide provider of process control technologies for improving
productivity in semiconductor and other advanced manufacturing processes, today
announced that its Board of Directors elected Leo Berlinghieri to the position
of President and Chief Operating Officer. He previously served as Vice President
and Chief Operating Officer. Mr. Berlinghieri continues to report to John R.
Bertucci, who resigned his position as President and continues as Chairman and
Chief Executive Officer.

      Mr. Berlinghieri retains responsibility for all product groups, global
sales and service, marketing, technology, and global business operations,
including quality, manufacturing, and supply chain management.

      Mr. Berlinghieri, 50, has served as Vice President and Chief Operating
Officer since July 2003 after serving as Vice President, Global Sales and
Service since 1995. He has had responsibility for operating positions in
manufacturing, supply chain management and customer service during his 23 years
with MKS.

      John Bertucci, Chairman and Chief Executive Officer, said, "Leo's
demonstrated skills in leading our talented management team have contributed to
MKS' growth, as we have taken the next steps in partnering with our customers to
help improve their process productivity and profitability. The Board and I are
pleased to recognize his leadership abilities and look forward to his continued
contributions in this dynamic business environment."

      MKS Instruments, Inc. is a leading worldwide provider of process control
solutions for advanced manufacturing processes such as semiconductor device
manufacturing; thin-film manufacturing for flat panel displays, data storage
media, architectural glass and electro-optical products; and technology for
medical imaging equipment. Our

instruments, components and subsystems incorporate sophisticated technologies to
power, measure, control, and monitor increasingly complex gas related
semiconductor manufacturing processes, thereby enhancing our customers' uptime,
yield and throughput, and improving their productivity and return on invested
capital.

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