A new type of cation-exchange membrane was prepared by grafting acrylic acid onto a microporous polyethylene membrane by use of plasma-grafted polymerization technique. The facilitated transport of CO2 through the plasma-grafted membrane containing ethylenediamine was studied. The obtained membrane showed remarkably high CO2 permeability and selectivity of CO2 over N2. When the CO2 partial pressure in the feed gas was 0.047 atm, the CO2 permeability and the selectivity obtained were 1.0 X 10(-4) cm3 cm-2 s-1 cmHg-1 and 4700, respectively. These efficiencies were much higher than those of conventional polymer membranes, and were also superior to efficiencies of ion-exchange membranes containing a carrier, reported so far. These excellent results are probably attributable to the high ion-exchange capacity and the high water content. The newly prepared membrane was stable.