A series of Nafion films with both carboxylic and sulfonic acid functional groups were prepared and tested for water/nitric acid permselectivity. Unlike the commercially available Nafion 90209 membrane, which is comprised of a sulfonate ionomer laminated to a carboxylate ionomer, the membranes in the present study contained carboxylate groups distributed among sulfonate groups, rendering them fundamentally different. The membranes were examined in both pervaporation and reverse osmosis modes of operation. Using pervaporation, the water/acid separation factor increased by 3 orders of magnitude, and the water permeability decreased by 3 orders of magnitude as carboxylate content increased from 0 to 30%. Both flux and selectivity increased with an increase in temperature and/or a decrease in feed concentration. In all instances, the separations achieved in pervaporation were less than those predicted for a flash separation. Using a liquid feed pressurized from atmospheric pressure to the point of mechanical membrane failure, no separation of water/acid was achieved at any feed pressure. Experimental results suggest that the carboxylate functional group rejects nitric acid by Donnan exclusion of the nitrate anion and by restriction of proton mobility, but only in a partially dry state, such as that created by a hard vacuum. As such, bifunctional carboxylate/sulfonate Nafion membranes may be useful for the dehydration of dilute nitric acid when used in pervaporation.