Reversible adsorption on zeolite molecular sieve material allows selective removal of carbon dioxide (CO2) from spacecraft air without the use of expendables. The four-bed molecular sieve (4BMS) CO2 removal subsystem chosen for use on space station is based on proven Skylab technology and provides continuous CO2 removal from the cabin atmosphere and concentration for further processing downstream or venting overboard.A 4BMS subsystem has also been chosen to remove CO2 from air in the Systems Integration Research Facility (SIRF) at NASA/Johnson Space Center (JSC). After installation in the SIRF in 1992, the subsystem underwent extensive testing in which cycle time, process air flow rate, and process air inlet CO2 composition were varied. In order to obtain performance data required for integration, the subsystem was operated under both nominal and off-nominal conditions. Results of this testing are presented.For future missions, alternative subsystem designs and operational modes for a molecular sieve-based CO2 removal subsystem may be more desirable than the baseline 4BMS subsystem, depending on mission drivers. In addition, development of new adsorbent materials may result in significantly lower power and weight penalties for a molecular sieve CO2 removal system. Alternative designs to the existing 4BMS configuration, which utilize existing hardware components and were considered during the space station redesign effort, are presented and discussed.