Integrally skinned asymmetric cellulose acetate membranes made by the wet phase inversion for removal of CO2 from natural gas were investigated. The membrane was cast with the membrane-forming systems of cellulose acetate-acetone and quench media, such as methanol, ethanol and isopropanol, respectively, without heat-treating and multistage exchange process. By means of evaluation on separating characteristics of the membrane for CO2/CH4, observation of morphologies by scanning electron photomicrographs and analysis of the phase diagrams on the membrane-forming systems, it has shown that the membrane-forming system of cellulose acetate-acetone-methanol is quite suitable to prepare integrally skinned asymmetric cellulose acetate membranes for gas separation with good selectivity CO2/CH4 = 30 and flux coefficient = 2.4 x 10(-5) cm(3)/cm(2) - s - cm Hg.