Facilitated transport of CO2 through a thin solution-cast perfluorosulfonic acid ionomer membrane has been studied. The CO2 carrier was monoprotonated ethylenediamine and was immobilized in the ionomer membrane by electrostatic force. The cast membrane with membrane thickness 22 mu m showed much higher CO2 permeance than the commercial Nafion(R) 117 membrane of thickness 186 mu m. However, selectivity of CO2 over N-2 was lower in the cast membrane. The effects of the membrane thickness and annealing temperature in the membrane preparation on membrane performance were investigated. CO2 permeability and the selectivity decreased with decreasing membrane thickness and with increasing annealing temperature. Dynamic mechanical studies were done to study the membrane structures. The cast membrane annealed at 200 degrees C showed two peaks similar to the commercial membrane. In contrast to this, the peak assigned to the polar region was small in the cast membrane annealed at a lower temperature, which suggested that clear clusters do not form in the membrane.