Changes in the electronic states of platinum-cobalt (Pt-Co) alloy catalysts through potential cycling between 0.6 and 1.0 V were investigated by X-ray photoemission spectroscopy (XPS) using synchrotron radiation. The electrochemical surface area loss and the particle size growth of the Pt catalyst were larger than those of the Pt-Co alloy catalyst. Pt 4f XPS spectra of the Pt-Co alloy catalyst do not show any change through the potential cycling, indicating that most part of Pt is stable during the potential cycling. Larger amount of Pt(OH)(2) existed in the initial MEA of the Pt catalyst than the Pt-Co alloy catalyst, indicating that the Pt catalyst has a tendency to be oxidized. The Pt(OH)(2) decreased and metallic platinum increased in the cycle-tested MEA, suggesting that the Pt(OH)(2) dissolved and re-deposited as metallic states. The oxidation tendency explains the less durability of the Pt catalyst than the Pt-Co alloy catalyst. Co 2p XPS spectra imply that cobalt is absent on the surface of the catalyst particles and the Pt skin layer is thicker than 1.4 nm (4 mono-layers). The absence of the cobalt oxide in the cycle-tested MEA demonstrates that the Pt-Co core under the Pt skin layer is stable during the potential cycling. (C) 2011 Elsevier B.V. All rights reserved.