Exhibit 99.1


(MKS LOGO)


                                                           FOR IMMEDIATE RELEASE

                                                                        CONTACT:
                                                                  Ronald Weigner
                                       Vice President & Chief Financial Officer
                                                                    978.975.2350

                      MKS INSTRUMENTS ANNOUNCES PRICING OF

                         PUBLIC OFFERING OF COMMON STOCK

   Andover, Mass., January 15, 2004, MKS Instruments, Inc. (NASDAQ: MKSI), a
leading worldwide provider of process control technologies for improving
productivity in semiconductor and other advanced manufacturing process
environments, today announced that it has set the price for the public offering
of 4,000,000 shares of its common stock at $26.25 per share. The offering
includes 1,142,857 shares offered by MKS in a new financing and 2,857,143 shares
offered by certain stockholders. Net proceeds to the Company are estimated at
$28.5 million. MKS will not receive any proceeds from the sale of shares held by
selling stockholders.

    Merrill Lynch & Co. is the book-running manager for the offering, with
J.P. Morgan Securities Inc., Adams, Harkness & Hill, Inc., and Needham &
Company, Inc. acting as co-managers.  The underwriters have an option to
purchase up to 171,429 additional shares of common stock from the Company and
up to 428,571 additional shares of common stock from the selling stockholders
to cover over-allotments, if any.

   MKS currently intends to use the net proceeds from any sale of MKS shares for
general corporate purposes, including working capital, product development and
capital expenditures, as well as potential acquisitions.

   Copies of the prospectus supplement can be obtained from the Prospectus
Department of Merrill Lynch, Pierce, Fenner & Smith Incorporated, 4 World
Financial Center, New York, NY 10080; the Prospectus Department of J.P.
Morgan Securities, Inc., 277 Park Avenue, New York, NY 10172; Adams, Harkness
& Hill, Inc., 60 State Street, 6th Floor, Boston, MA 02109; Needham &
Company, Inc., 445 Park Avenue, New York, NY

10022; or from the Vice President and Chief Financial Officer, MKS Instruments,
Inc., Six Shattuck Road, Andover, MA 01810.

   This press release shall not constitute an offer to sell or the solicitation
of an offer to buy, nor shall there be any sale of these securities in any
jurisdiction in which such offer, solicitation or sale would be unlawful prior
to the registration or qualification under the securities laws of any such
jurisdiction.

   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, optical 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 failure of MKS and acquired
companies to realize the anticipated benefits of their combined businesses, the
challenges and risks involved with integrating the operations of MKS and
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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