Perkin-Elmer is providing the Atmosphere Composition Monitor (ACM) for the Space Station Freedom Environmental Control and Life Support System (ECLSS) being developed under Work Package 1 by the prime contractor, Boeing Aerospace & Electronics (BA&E), a division of The Boeing Company. In addition to ECLSS, BA&E will build Space Station Freedom's living and working modules as Work Package 01 under prime contract with NASA's Marshall Space Flight Center. Prior to production of flight hardware the ECLSS system will be tested in the Predevelopment Operational System Test (POST). “Predevelopment” or POST equipment must meet flight hardware functional requirements, but not size, weight, and power goals. The POST ACM consists of the following major modules: a Major Constituent Analyzer, a Trace Contaminant Monitor, a Carbon Monoxide Analyzer, a Particle Counter Monitor, a Sample Acquisition System and a Computer System. The Predevelopment ACM is designed for automated operation; operator intervention is required only for maintenance. Instrument performance is verified through periodic measurement of reference gas standards.