The effect of crosslinking by UV irradiation on the gas permeation properties of thin films (thickness less than or equal to 1 mu m) made from two benzophenone-based polyarylates were examined. In addition to the permeation response to UV crosslinking in these two polymers, the effects of crosslinking on the rate of physical aging was also explored. The sequence of physical aging and crosslinking, as well as reversal of the aging process was studied in order to separate the similar effects of aging and crosslinking. The results show that crosslinking very thin films can greatly improve the long-term performance of membranes when compared to noncrosslinked films of similar thickness.