In this article, we report the influence of the polyimide molecular weight (1.2 x 10(5), 2.6 x 10(5), and 4.1 x 10(5)) on the structure and the gas permeation properties of asymmetric polyimide membranes made by the dry wet phase-inversion process. The apparent skin layer thickness of the asymmetric membrane increased with increasing molecular weight, and the thicknesses of the membranes prepared from the three polyimides with a casting polymer solution containing 8.0 wt % butanol were 132, 350, and 739 nm, respectively. That is, the gas permeance in the asymmetric membranes increased with decreasing molecular weight. In contrast, the gas selectivity of the asymmetric membranes did not depend on the skin layer thickness. The solvent evaporation in the dry phase-inversion process and the nonsolvent diffusion in the dry process were important factors that determined the formation of the asymmetric membrane. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 105-112, 2010