The effects of both geometric (Pt-Pt bond distance) and electronic (Pt d-band vacancy) factors on the electrocatalytic activity for the oxygen reduction reaction (ORR) of carbon supported Pt and Pt-Co alloy catalysts were investigated by Xray diffraction and X-ray absorption spectroscopies in conjunction with electrochemical measurements in proton exchange membrane (PEM) fuel cells. In the presence of cobalt, a decrease in the Pt-Pt bond distance and an increase in the metal particle size were observed. In PEM fuel cells the ORR activity on all the Pt-Co/C catalysts was higher than that oil Pt/C. A poor fit between the ORR activity and the Pt-Pt bond distance was found. Conversely, the ORR activity presented a linear relationship with the integrated intensity of the Pt L-3 edge at 1.1V, related to the Pt d-band vacancy. This is because this parameter takes into account both alloying (Pt-Pt bond distance) and particle size effects. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.