In order to clarify the oxidation mechanism of carbon materials in polymer electrolyte fuel cells (PEFCs), electrochemical oxidation of highly oriented pyrolytic graphite (HOPG) was investigated in sulfuric acid solution at less than 1.0 V vs normal hydrogen electrode. Surface oxidation was accelerated at 1.0 V, whereas oxidation kinetics seemed to be slow at 0.6 V. In situ electrochemical atomic force microscopy clearly showed that hillock structure was developed with a formation of C-O bonding on the basal plane of HOPG in the early stage of the oxidation process. Hence, the hydrolysis reaction to produce alcohol and other functional groups was considered as the initiation reaction for electrochemical oxidation of HOPG. Accumulation of the surface strain by the formation of hillocks may lead to development of the oxidized layer. The oxidation kinetics of further process accompanied with an evolution of CO or CO2 could be slower than that of the hydrolysis reaction, which might be ascribed to the low electron conductivity of the oxidized HOPG surface. The mechanism for electrochemical oxidation of HOPG contributes an elucidation of carbon oxidation reactions in PEFCs. (c) 2007 The Electrochemical Society.