A comprehensive overview of the properties of Nafion membranes under electrolysis conditions is presented here to help evaluate and design high-pressure operating proton exchange membrane (PEM) electrolyzers. First, the properties of Nafion membranes are reviewed and summarized based on literature data for PEM electrolysis conditions. Second, the solubility and diffusivity of H(2) and O(2) in liquid water and the PEM are reviewed based on literature data. Finally, the relationship between current density and the impurity of gases is reviewed and discussed. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.