Asymmetric membranes for gas separation were fabricated using the layer-by-layer adsorption process based on the spontaneous self-assembly of alternating layers of cationic and anionic polymers an porous and solid support membranes. The support membranes were first dipped in a dilute solution of poly(allylamine) (a polycation) followed by dipping in a dilute solution of poly (styrenesulfonate) (a polyanion). Repeating this process, 40 polymer layers were deposited on porous poly(propylene) membranes (Celgard 2400) and 100 layers on solid dimethyl silicone membranes. Gas-transfer experiments at several temperatures indicated reduced mass permeabilities due to the adsorbed films. Permeabilities of pure CO2 and N-2 through coated Celgard samples were virtually equal at all temperatures indicating dominance of Knudsen diffusion through micropores in the film. Permeabilities in coated silicone membranes indicate higher CO2/N-2 selectivity than the bare membrane at elevated temperatures. Microphotography indicated the presence of breaks in the polyion complex coating on the silicone membrane. Fabrication of gas transfer membranes via self-assembly of ionic polymers offers the possibility of designing highly selective membranes as well as the ability to control the thickness and architecture of films at the molecular level.