Oxidative. corrosion of the surface of highly oriented pyrolytic graphite (HOPG) and effects of platinum and oxygen on this corrosion were investigated at the atomic scale by using electrochemical scanning tunneling microscopy (EC-STM) to understand the degradation of the carbon Supports in the cathodes of polymer electrolyte fuel cells. Morphological changes within several layers of the HOPG surface were observed under fuel-cell operating conditions. The EC-STM observations revealed that the corrosion occurred first at step edge (i.e., at defect sites on HOPG surfaces forming graphitic oxides) and then progressed toward terraces, indicating that the oxidation along the in-plane direction was fast and the basal plane was rather resistant to oxidative corrosion. We also found that platinum and oxygen at the HOPG surface strongly enhance the oxidative corrosion of HOPG. Oxygen species formed at platinum/carbon interface and oxygen atoms formed by dissociative adsorption would likely enhance the oxidation of graphitic-oxide intermediate species. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3187924] All rights reserved.