Carbon dioxide removal from the Space Station Freedom atmosphere is an essential part of crew life support. Freedom must utilize carbon dioxide removal systems to prevent crew asphyxiation. This paper describes the predevelopment operational system test (POST) four-bed molecular sieve carbon dioxide removal assembly, its operation, and its key components.Many approaches are available to effect carbon dioxide removal, and include both regenerative and non-regenerative methods. To minimize Freedom logistics support and to close the cabin oxygen loop, regenerative systems will be relied upon.The Freedom carbon dioxide removal assembly will selectively remove carbon dioxide from an air supply stream, preventing carbon dioxide accumulation within the cabin, and then concentrate it for downstream processing in a carbon dioxide reduction system where oxygen eventually is recovered.Freedom will utilize a regenerative four-bed molecular sieve system for the carbon dioxide removal assembly. The four-bed system uses the principle of adsorption to selectively remove and concentrate carbon dioxide. The technology required for the four-bed system is well established and was proven in space flight during the Skylab program.Under contract to the Boeing Aerospace and Electronics Company, the AiResearch Los Angeles Division of Allied-Signal Aerospace Company designed and fabricated a four-bed molecular sieve carbon dioxide removal assembly. This assembly will be tested under the POST program to demonstrate closed-loop operation.The four-bed system fabricated for the POST program is functionally identical to and meets the performance requirements of the Freedom flight system. The POST system hardware has characteristics similar to the flight system currently being designed for Freedom.