New facilitated transport membranes which selectively permeate carbon dioxide from hydrogen and methane have been prepared and examined under a variety of conditions. Membranes consist of melts of the salt hydrates tetramethylammonium fluoride tetrahydrate, [(CH3)(4)N]F . 4H(2)O, or tetraethylammonium acetate tetrahydrate, [(C2H5)(4)N]CH3CO2 . 4H(2)O, immobilized in films of Celgard 3401(R). Operating at 50 degrees C, both membranes exhibited CO2 permeabilities which increased with decreasing feed partial pressure of CO2, characteristic of a facilitated transport membrane. Selectivities of CO2/H-2 and of CO2/CH4 increased with decreasing feed pressure since H-2 and CH4 permeances were independent of feed pressure. Selectivities of CO2/H-2 were disappointingly low due to permeation of H-2 through the dense phase of Celgard(R). With some difficulty, the microporous support was eliminated by construction of membranes consisting of liquid [(CH3)(4)N]F . 4H(2)O on the surface of a film of poly( trimethylsilylpropyne). Selectivities of CO2/H-2 as high as 360 were then observed at low feed partial pressures of CO2. Modeling of membrane properties is consistent with permeation of CO2 by a facilitated transport mechanism with reasonable derived diffusivities for the chemically bound CO2 carrier species.