Platinum oxidation of Pt-Co alloy catalysts for polymer electrolyte fuel cells was investigated for a series of Pt-Co alloy catalysts with different specification. The chemical state of platinum evaluated by soft X-ray photoemission spectroscopy was compared with the electrochemical properties to elucidate the origin of catalyst degradation. Increase in the particle size of Pt-Co alloy catalysts caused the decrease in the concentration of platinum hydroxide and improved the catalyst durability. Applying potential cycling below 1.0 V. only platinum hydroxide was observed, while platinum oxides, PtO and PtO2, appeared after potential cycling up to 1.2 V. The peak shift of Pt 4f spectra after the potential cycling implies that these platinum hydroxide and oxide are dissolved and deposited on another platinum catalyst in a reduced metallic state, which causes the catalyst degradation. (C) 2012 Elsevier B.V. All rights reserved.