Asymmetric polycarbonate (PC) membranes were prepared by the wet phase inversion method. The formation of asymmetric oxygen enrichment membranes was studied with respect to the type of coagulation medium, polycarbonate concentration, and nonsolvent content in the casting solution. The gas pressure-normalized fluxes (P/l) and membrane morphologies for the prepared membranes were investigated. The rate of liquid-liquid phase separation decreases with increasing alcohol molar volume of the coagulation medium. The gas separation performance for nonsolvent-added polycarbonate membranes was better than that of the membrane without additive with a pressure-normalized flux of 0.034 X 10(-6) cm(3)(STP)/cm(2)-sec-cmHg and an O-2/N-2 selectivity of 6.46. An oxygen pressure-normalized flux of 3 X 10(-6) cm(3)(STP)/cm(2)-sec-cmHg and an O-2/N-2, selectivity of 4 can be obtained for the membrane prepared from the CH3OH/CH2Cl2/PC (coagulation medium/solvent/polymer) system with nonsolvent n-propanol added in the casting solution.