A study combining theoretical predictions and experimental measurements was made to gain an understanding of the beneficial effect of alloying cobalt into platinum for electroreduction of oxygen. Carbon-supported Pt3Co catalyst particles were characterized by X-ray diffraction spectroscopy and X-ray absorption near-edge structure, which gave evidence for a surface layer composed of Pt, called the Pt skin. Electrochemical measurements were made in 1 M trifluoromethane sulfonic acid with a rotating ring disk setup. Cyclic voltammetry showed significantly less oxide formation in the > 0.8 V range over the skin on the alloy compared to nonalloyed Pt. Tafel plots showed a 50-70 mV reduction in overpotential for O-2 reduction over the Pt skin. The Vienna Ab Initio Simulation Program was used for calculating H2O and OH adsorption bond strengths on the Pt skin on Pt(3)Cos(111) for comparison with prior work with the Pt(111) surface. The bond strength variations were used to estimate the shift in reversible potential for OHads formation from H2Oads oxidation. A shift of 80 mV was found, which indicates that an increase in the reversible potential for OHads formation correlates with the decrease in overpotential for O-2 reduction over the Pt skin on Pt3Co nanoparticles. © 2005 The Electrochemical Society. All rights reserved.