The performance of ion-exchange membranes under pressure differential conditions is important for a number of separations and for fuel cell applications. Fluid transport properties under pressure are also key to understanding electrochemically-driven pumping and pressurization systems. The physicochemical properties and performance of perfluorosulfonic acid membranes varies considerably with conditioning treatment. In this study, hydraulic permeability and Water uptake are measured as it function of membrane treatment for Nafion (R) membranes that are contacted with aqueous electrolyte solutions. Several conditioning/contacting solution treatments are evaluated including exchanged cation. manner of cation exchange, anion effect, and heat treatment. For Nafion (R) 117 membranes. these treatments resulted in water content ranging front 3.8 to 20.2 water molecules/fixed sulfonate site and hydraulic permeability ranging from 3.78 x 10(-16) to 1.79 x 10(-13) m/s Pa. similar to water uptake, a systematic decrease in hydraulic permeability is observed with the alkali series Li+ > Na+ > K+. Interestingly. the as-received H+ form and the Li+ form have nearly identical water content but the permeability of the H+ form is a factor of 6-7 higher than the Li+ form, regardless of heat treatment. Evaluation based on an equivalent pore model and Poiseuille flow shows good agreement for pore radii ranging from 1.0 to 2.0 nm. (c) 2006 Elsevier B.V. All rights reserved.