A series of high-molecular weight condensation polyimides was evaluated to determine the effect of polymer molecular structure on the transmission rate of oxygen, carbon dioxide, and water vapor. The polyimide films were prepared from either 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) or pyromellitic dianhydride (PMDA) with various diamines. The study shows that molecular structure had a strong influence on gas transmission rates with results for some films varying three orders of magnitude from that of other polyimide films. In general, the BTDA series of polyimides had overall lower gas transmission rates than the PMDA-derived series. Polymers prepared with meta-oriented diamines characteristically displayed lower gas transmission than those prepared with para-oriented diamines.