A major goal of polymer electrolyte fuel cell (PEFC) efforts is an understanding of how process conditions and material composition affect sources of overpotential in an operating cell. Limited ionic conductivity within the catalyst layer is one such source of overpotential. We report the results of a study of the impact of processing conditions, specifically hot pressing and boiling in acid, on the ionic and electronic resistivities of catalyst layers made from platinized VULCAN XC-72 (XC-72) carbon. Our results show that the greatest gains in ionic conductivity for a PEFC catalyst coated membrane comes from acid exchange of the active layer in tetrabutylammonium form. We also probe the dependence of ionic and electronic resistivities of catalyst layers on their chemical composition. We determine the ionic and electronic conductivities of surface modified unplatinized XC-72 carbon with phenyl sulfonic acid of varying weight percentage. Nafion composites with the modified materials display an increase in ionic conductivity of more than an order of magnitude when compared to a composite layer consisting of plain XC-72 and Nafion. (C) 2003 The Electrochemical Society.