[MKS INSTRUMENTS, INC. LOGO]

                                                                    Exhibit 99.1



                                                           FOR IMMEDIATE RELEASE

                                                     INVESTOR RELATIONS CONTACT:
                                                                  Ronald Weigner
                                        Vice President & Chief Financial Officer
                                                                    978.284.4446


          MKS INSTRUMENTS ACCELERATES VESTING OF CERTAIN STOCK OPTIONS

Wilmington, Mass. -- January 11, 2005 - MKS Instruments, Inc. (NASDAQ: MKSI), a
leading worldwide provider of process control technologies for semiconductor and
other advanced manufacturing processes, announced that on January 7, 2005, it
accelerated the vesting of certain unvested options awarded to its employees and
officers under its stock option plans. The acceleration applies only to those
options with an exercise price of $23.00 or greater. The closing sale price of
MKS stock on the Nasdaq National Market on January 7, 2005 was $16.03. As a
result of this action, options to purchase approximately 1.6 million shares
became exercisable on January 7, 2005.

Under the recently issued Financial Accounting Standards Board Statement No.
123R, "Share-Based Payment" ("FAS 123R"), the Company will be required to apply
the expense recognition provisions under FAS 123R beginning July 1, 2005. The
Company believes that accelerating the vesting of the identified stock options
will reduce the Company's compensation charge in periods subsequent to June 30,
2005.

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 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 possibility that
FAS 123R will be changed, amended or interpreted, thereby changing the Company's
current assessment of the impact the adoption of FAS 123R will have on the
acceleration of the vesting of stock options, as






well as other risk factors detailed in MKS' filings with the Securities and
Exchange Commission, including its most recent Quarterly Report on Form 10-Q.
Readers are directed to those filings for a discussion of these 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.