An Atmosphere Composition Monitor (ACM) is being developed for monitoring the atmospheric composition in the Space Station Freedom. The instrument is part of the Environmental Control and Life Support System (ECLSS) that is being developed by Boeing Aerospace & Electronics.*Monitoring the atmospheric composition in the Space Station Freedom will be essential to ensure crew health. The ACM measures the major atmospheric constituents to provide feedback for the nitrogen/oxygen replenishment control. It also measures carbon monoxide, particulates, and trace organic gases resulting from material outgassing, chemical leaks or spills, metabolic byproducts, and possibly electrical equipment malfunction. Information provided by the ACM can be used to detect cabin air leakage and to verify the proper function of the ECLSS Atmospheric Revitalization system. Data on trace organic contaminants ensures that toxic compounds are maintained below their Spacecraft Maximum Allowable Concentration (SMAC) levels to ensure the well-being of the crew.The ACM consists of the following major instruments: a single focusing mass spectrometer to monitor major air constituents; a gas chromatograph/mass spectrometer to detect trace organic contaminants; a non-dispersive infrared spectrometer to determine carbon monoxide concentration; and a laser particle counter for measuring particulates in air.The conceptual design of the ACM has been described previously (1). In this paper, we will present detailed design considerations and test results for selected subsystems. These include: the sampling subsystem design; the electrometer design to achieve sub-picoampere detectability; and development of the gas chromatograph method and a novel mass spectral detection algorithm.