Ion beam irradiation has been widely used to modify the structure and properties of membrane surface layers. In this study, the gas permeability and selectivity of an asymmetric polyimide membrane modified by He ion irradiation were investigated using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 degrees C. Specifically, we estimated the effects of the gas diffusion and solubility on the gas permeation properties of the asymmetric membranes with the carbonized skin layer prepared by ion irradiation. The asymmetric polyimide membranes were prepared by a dry-wet phase inversion process, and the surface skin layer on the membrane was irradiated by He ions at fluences of I X 10(15) to 5 x 10(15) ions/cm(2) at 50 keV. The increase in the gas permeability of the He+-irradiated asymmetric polyimide membrane is entirely due to an increase in the gas diffusion, and the gas selectivity increases of the membranes were responsible for the high gas diffusion selectivities. (c) 2006 Wiley Periodicals, Inc.