Permeability of H2, CO2, O2, CO, N2 and CH4 in polyimides prepared by polycondensation of equimolar amount of 3,3’,4,4’-benzophenone tetracarboxylic dianhydride (BTDA) or BTDA/2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (1:1 or 1:3 molar ratio) with 2,4,6-trimethyl-1,3-phenyl-ene-diamine was measured at 35-degrees-C to investigate the effect of photocrosslinking on permeability and permselectivity. The permeability coefficients decreased with increasing UV-irradiation time as a result of crosslinking. The decreases in the permeability coefficients were attributed to a change in diffusivity, while solubility was not greatly affected. Permselectivity of photocrosslinked BTDA-containing polyimides is controlled by the diffusivity selectivity and is higher than that of the polyimides with similar permeability. The largest change in selectivity was obtained for the H2/CH4 gas pair which has a large difference in the molecular sizes of the constituents. The fact that the selectivity for the H2/CH4 separation increased by a factor of 50 after 30 min UV-irradiation with a decrease in H2 permeability by a factor of 5 is especially interesting from a membrane separation standpoint.