Integrally skinned asymmetric gas separation membranes were prepared by (i) dry, (ii) wet, and (iii) dry/wet phase inversion processes. The membranes were cast from a polysulfone/methylene chloride/1,1,2-trichloroethane/2-methyl-2-butanol casting system. Wet and dry/wet phase inversion membranes were quenched in methanol. Membranes made by dry/wet phase inversion using convective evaporation showed optimum gas separation performance. The average O2/N2 and He/N2 selectivities of these membranes were within 85% of those determined for a dense, solution-cast polysulfone film, suggesting that the ultrathin skin layers were essentially defect free. The average apparent skin layer thickness of all samples tested was 270 Å. Scanning electron photomicrographs revealed that optimum membranes made by dry/wet phase inversion consist of an ultrathin skin layer, a tightly packed nodular transition layer, and an open-cell, sponge-like substructure. Dry/wet phase inversion membranes prepared by free-standing evaporation resulted either in high flux and low selectivity membranes or essentially defect-free membranes with fluxes lower than those made by convective evaporation. Dry-phase inversion membranes exhibited extremely low gas fluxes due to thick (17.5 μm) skin layers. On the other hand, wet phase inversion membranes showed O2/N2 selectivities < 1, indicating that gas transport was determined by pore flow through skin layer defects.