A CO2-selective membrane has many applications, including the purification of synthesis gas to obtain high purity hydrogen for fuel cells and the separation of carbon dioxide from flue gas for the greenhouse gas sequestration. CO2-selective membranes consisting of dimethylglycine (DMG) salts as the mobile carriers and polyethylenimine as the fixed carrier in crosslinked poly(vinyl alcohol) were synthesized. Based on the carbon dioxide flux data as a function of membrane thickness, mass transfer resistances due to the reactions at the feed-side and permeate-side interfaces of the membrane were determined. Thus, the "true" permeability, which is independent of membrane thickness, was obtained. Also investigated were the effects of water content and mobile carrier concentration in the membrane on the transport properties of carbon dioxide. Carbon dioxide flux/permeability and carbon dioxide/hydrogen selectivity increased as the water content and mobile carrier concentration increased. Both permeability and selectivity increased as temperature increased. The permeation activation energy determined for the membrane with DMG-Li was much lower than that with DMG-Na.