The effect of post-synthetic treatment with oxalic acid was investigated on the pervaporation performance of mordenite membranes for the separation of a water/ethanol mixture. It was demonstrated that the water/ethanol separation factors were enhanced from 6–44 to higher than 10,000 after as-synthesized mordenite membranes were hydrothermally treated in an oxalic acid solution under appropriate conditions. The change in the pervaporation performance of mordenite membranes upon post-treatment with oxalic acid was discussed.