A system comprised mainly of 90 electrochemical cells has been designed for use as a CO2 concentrator in a manned spacecraft. Cabin air, with a CO2 partial pressure of about 3 mm Hg is passed across the cathode of an oxygen-hydrogen fuel cell. It is concentrated through the carbonate electrolyte and expelled into the hydrogen-filled anode cavity.The total system, as well as the individual cell design, is described. Experimental results are shown for the full (90 cell) system and also for smaller scale (1 and 3 cell) tests. Excellent consistency among the tests was found.A steady state analytical model has been developed and numerical simulations of the system have been carried out. The model consists of two parts. The first part is established based on the rate equations which govern each of the processes controlling the CO2 transfer in the system. It is a non-linear boundary value problem which is solved by a shooting method. Most of the parameters needed for carrying out the numerical simulation of this part are evaluated from independent physical and chemical data. Rates of CO2 transfer are calculated for inlet CO2 partial pressure of 0 to 14 mm Hg and current density from 5 to 40 amps per square foot. The simulation results are shown to fit the test data of concentrators of different configurations within experimental error.The second part, which simulates steady-state heat and water transfer in the system, is established based on simple heat and mass transfer equations. It takes into account the cooling method used in the concentrator. The process air temperature profiles and outlet air dewpoints calculated by the model are shown to fit the test data within experimental error.The combined model has been used to compute the detailed concentration distributions in the bulk electrolyte. These results are used to predict incidence of precipitation in the electrolyte as well as to calculate the pH. This capability is then utilized to confirm the adequacy of the control techniques used in the concentrator operation.