Electrochemical characterization of a series of membranes based on polystyrenesulfonic acid grafted unto ethylenetetrafluoroethylene (ETFE-g-PSSA) was determined by chronoamperometry with a microdisk electrode using a solid-state electrochemical cell at 100% relative humidity and over a range of temperatures. The oxygen diffusion coefficient and permeability increased with decreasing equivalent weight (EW). Oxygen solubility marginally decreased. These observations can be explained on the basis of increasing water content with decreasing EW. Proton conductivity also increased with water content and does not directly correlate with proton concentration within the membrane,