In this paper, diffusion and electro-osmotic drag of water across Nafion(R) membranes in the presence of HCl are characterized. For all the measurements, one side of the Nafion membrane was in contact with liquid water and the other side with gaseous anhydrous HCl. To characterize diffusion of water, the open-circuit flux of water across a catalyst-coated Nafion 115 membrane was measured as a function of HCl flow rate and temperature at a constant cell pressure of 1 atm. Due to the nature of varying driving force for diffusion as a function of HCl flow rate, the experimental data was analyzed in conjunction with a mathematical model. The mathematical model accounts for condensation of water and is used to calculate the concentration of liquid hydrochloric acid in contact with the membrane. The mathematical model presented here is general and can be applied to the characterization of water transport across Nafion membranes in the presence of any gas that is soluble in water. In the case of HCl, at low inlet flow rates (<1500 cm(3)/min, STP), the diffusion of water across the membrane is primarily governed by the diffusional limitations of HCl in the condensed phase. At high flow rates (>3000 cm(3)/min, STP), the flux of water is a constant and depends on the saturation solubility of HCl in the condensed liquid phase. To measure the electro-osmotic drag parameter, the net flux of water across the membrane was measured as a function of the applied current density at high HCl flow rates (i.e., uniform water flux). At 80degreesC, it was found that 3.8 mol of water per mole of protons are transported from the anode to the cathode.