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                                                           FOR IMMEDIATE RELEASE

                                                            Contact: Ron Weigner
                                      Vice President and Chief Financial Officer
                                                                    978.284.4446


               MKS INSTRUMENTS REVISES THIRD QUARTER 2004 OUTLOOK

     Wilmington, Mass. - September 16, 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 sales for the third quarter ending September 30,
2004 are expected to be below previous guidance for flat sequential growth due
to lower than anticipated orders for the semiconductor capital equipment market
along with some stretched out delivery requests and order push outs. Third
quarter 2004 sales are now expected to range from $135 to $140 million, or
approximately 8 to 11 percent below second quarter 2004 sales of $151.6 million.
MKS' third quarter 2003 sales totaled $81.6 million.

     Based on this lower sales volume, third quarter 2004 GAAP earnings could
now range from $0.17 to $0.21 per diluted share. Non-GAAP earnings, which
exclude amortization of acquired intangible assets of approximately $3.7
million, could range from $0.24 to $0.28 per diluted share in the third quarter
of 2004.

     John Bertucci, Chairman and Chief Executive Officer, said, "We have
achieved significant sales growth, and our year-over-year growth rate for the
nine months could range from 78 to 80 percent. Over the same period, we estimate
our semiconductor and thin-film sales growth rate at 101 percent or well above
the forecast growth rate of 66 percent for our served semiconductor front-end
equipment market. We believe this recent softening in order rates reflects our
customers' relatively flat near term outlook, which requires no further increase
in pipeline inventory and therefore has reduced demand for our products to a
level more closely aligned to our customers' production rates. Although our
visibility is limited, we currently anticipate that fourth quarter revenues
could be essentially flat and in line with our customers' outlook. We remain
focused on managing our costs and executing our strategies for increased
profitability at all sales levels."




     Management plans to release third quarter financial results and fourth
quarter outlook after market close on Tuesday, October 19, 2004, and to host a
conference call afterward at 5:00 p.m. (Eastern Time). A web cast of the
conference call will be available at www.mksinstruments.com.

     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.

     This release contains projections or other forward-looking statements
within the meaning of the Private Securities Litigation Reform Act of 1995
regarding MKS' future growth and the future financial performance of MKS. These
projections or statements are only predictions. Actual events or results may
differ materially from those in the projections or other forward-looking
statements set forth herein. Among the important factors that could cause actual
events to differ materially from those in the projections or other
forward-looking statements are the fluctuations in capital spending in the
semiconductor industry, fluctuations in net sales to MKS' major customers,
potential fluctuations in quarterly results, the challenges, risks and costs
involved with integrating the operations of MKS and any acquired companies,
dependence on new product development, rapid technological and market change,
acquisition strategy, manufacturing and sourcing risks, volatility of stock
price, international operations, financial risk management, and future growth
subject to risks. Readers are referred to MKS' filings with the Securities and
Exchange Commission, including its most recent Quarterly Report on Form 10-Q,
for a discussion of these and other important risk factors concerning MKS and
its operations. MKS is under no obligation to, and expressly disclaims any
obligation to, update or alter its forward-looking statements, whether as a
result of new information, future events or otherwise.



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