In this study, the gas diffusion and solubility of the asymmetric polyimide membrane irradiated by He ions were investigated using a high vacuum apparatus equipped with a Baratron absolute pressure gauge at 76 cmHg and 35 degrees C. Specifically, we estimated their effects on the gas permeation properties of the asymmetric membranes. 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 1 x 10(15) or 3 x 10(15) ions/cm(2) at 150 keV. We demonstrated that the gas diffusion had a significant influence on the gas permeability and selectivity of the He+-irradiated asymmetric membrane. (c) 2006 Elsevier Ltd. All rights reserved.