A photocrosslinkable polyimide membrane was prepared and investigated with regard to the effect of ultraviolet light irradiation (UV-irradiation) using a high-pressure mercury lamp on their gas permeabilities and permselectivities. Permeability and diffusion coefficients for O2, N2, H2, and CO2 were determined using the vacuum-pressure and time-lag methods. Sorption properties for carbon dioxide were determined to evaluate the changes in the free volume in the membranes by the irradiation. The apparent gas permeabilities decreased and permselectivity, particularly for H2 over N2, increased with increasing UV-irradiation time without a significant decrease in the flux of H2. They depended on the membrane thickness, suggesting asymmetrical changes in the membrane due to UV-irradiation.