The dependence of electrode proton resistivity on electrode thickness, Pt loading, ionomer loading, and ionomer equivalent weight (EW) in proton exchange membrane (PEM) fuel cell cathodes was investigated using a Pt/Vulcan catalyst. For uniform electrodes, the electrode proton resistivity is independent of the electrode thickness and Pt loading but depends on the ionomer loading and ionomer EW. There is a strong dependence on the ionomer EW when the ionomer/carbon weight (I/C) ratio is lower than 0.8. The electrode proton resistivity strongly depends on relative humidity (RH) and the density of -SO3H groups in the electrode. The electrode proton resistivity becomes nearly independent of ionomer EW in electrodes when high I/C ratios are used. At low I/C ratios and low RH levels, electrodes with 850 EW ionomer exhibit better performance than those with 1050 EW. On the contrary, 850 EW electrodes give lower performance under overhumidified conditions due to electrode flooding. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3435323] All rights reserved.