In this article, we have reported the influence of alcohol as a casting solution on the structure and the gas permeability of asymmetric polyimide membranes made by dry-wet phase inversion. The apparent skin layer thickness of the asymmetric membrane decreased with an increase in molecular weight of the alcohol, and the thicknesses of the membranes made from methanol, ethanol, propanol, and butanol were 250, 120, 61, and 31 nm, respectively. We found that X-12 as an interaction parameter of solvent-nonsolvent had a significant influence on the phase inversion occurring in the coagulant medium. On the other hand, the gas permeance and the gas selectivity in the asymmetric membranes increased with the increasing molecular weight of the alcohol. We believe that a more packed structure formed in the asymmetric polyimide membrane with a thinner surface skin layer is also responsible for the thickness-dependence of the gas selectivity obtained in this study. (c) 2007 Wiley Periodicals, Inc.