Two asymmetric polyimide membranes that have a thermal stability up to 550 K and good resistance to solvents were prepared. Some membrane-forming conditions such as coagulation solution, evaporation and immersion time were examined. Regarding the coagulation solution, a water-methanol mixture gives good membranes in eleven solutions tested. The permeabilities of two asymmetric polyimide membranes for CO2 and CH4 pure gases were determined at 25 degrees C and at applied pressures up to 0.25 MPa. The prepared polyimide membranes. 3,3 ',4,4'-biphenyltetracarboxylic dianhydride (BPDA)-4,4'-diaminodiphenyl ether (ODA) and 1,2,3,4-butanetetracarboxylic dianhydride (BDA)-4,4'-diaminodiphenyl ether (ODA) exhibited higher selectivity and permeability for CO2/CH4.