Two types of polyimide membranes; one crosslinkable and the other noncrosslinkable using ultraviolet light irradiation (UV irradiation), were prepared and investigated concerning the effect of UV irradiation on their gas permeabilities and selectivities. Permeability and diffusion coefficients for O-2, N-2, H-2, and CO2 were determined using the vacuum pressure and time lag method. Sorption properties for carbon dioxide were carried out to evaluate the changes in the free volume in the membranes due to the irradiation. In both membranes, permeability coefficients for all gases used in this study decreased and permselectivity, particularly for H-2 over N-2, increased with increasing UV irradiation time without a significant decrease in the flux of H-2. The coefficients depended on the membrane thickness, suggesting asymmetrical changes in both membranes due to UV irradiation. It was suggested by an attenuated total reflection (ATR) FTIR method and analysis of the gas sorption properties of the membranes that the physical changes due to UV irradiation at the irradiated side in both membranes significantly affected their gas permeation properties compared with the chemical changes, especially the crosslinking in the crosslinkable type. O 1997 John Wiley & Sons, Inc.