Abstract Water swelling behavior and permeation performance of CO 2 and N 2, CH 4, H 2 in polyvinylamine/polyvinylalcohol (PVAm/PVA) blend fixed-site-carrier (FSC) membrane were studied with respect to the relative humidity of the gases in the separation system. Two exponential fittings were presented to describe the two stage swelling behavior of the membrane. The effects of operating parameters on CO 2 separation performance were investigated and optimized. Interactions between gases in mixed gas separation were discussed. Experiments showed exponential dependencies of gas permeance on relative humidity, which could be explained by the dependence of gas diffusion on membrane swelling and the unique CO 2 facilitated transport membrane in this membrane. The reversible hydration reactions of CO 2 initiated by amino carriers in the water swelling membrane facilitate the CO 2 transport. CO 2 diffuses in the form of HCO 3 − in this membrane, resulting in both high CO 2 permeance and selectivity over other gases. A selectivity up to 45 and CO 2 permeance up to 0.55 m 3(STP)/(m 2 h bar) for CO 2–CH 4 separation, and a selectivity up to 160 and CO 2 permeance up to 0.83 m 3(STP)/(m 2 h bar) for CO 2–N 2 separation were documented.