By using water-miscible and water-immiscible solvents (N-methyl-2-pyrrolidone and methlyene chloride (MC)) in the casting solution to prepare poly(ether imide) (PEI) asymmetric membranes via a phase-inversion process, we have studied the flat membrane formation, membrane morphology, and gas separation performance in terms of PEI content, MC content, and standing time. In such a system, the immiscibility of MC with water plays a very important role in determining the final membrane morphology and performance. The influence of methylene chloride was shown in two aspects: (1) change of phase-inversion course and mechanisms; (2) post membrane structure change due to the interaction between PEI and methylene chloride. The silicone-coated PEI membranes exhibited a good membrane performance with O-2 permeance of (1.3-9.3) x 10(-6) cm(3)(STP)/cm(2). s . cmHg and O-2/N-2 selectivity of 5.2-6.4. It was found, from the gas permeation and membrane morphology studies, that the separation was mainly attributed to the cellular structures (closed cells) beneath the membrane surface skin.