Integrally skinned asymmetric cellulose acetate(CA) membranes made by the wet phase inversion for removal of CO2 from natural gas were investigated.The membrane forming system of CA acetone methanol proposed in this paper is quite suitable to prepare integrally skinned asymmetric CA membranes for gas separation with good separation factor α CO2/CH 4 =30 and flux coefficient ( P/L ) CO2 =1.8×10-8cm3(STP)/cm2·s·Pa at the pressure of 0 5MPa.Scanning electron micrographs revealed that optimum membranes consist of an ultrathin

essentially defect-free skin later(～200nm)and an open-cell

sponge-likesubstructure.