We focused on lactone formation during the electrochemical oxidation of highly oriented pyrolytic graphite (HOPG) as a model carbonaceous material for use in polymer electrolyte fuel cells in the potential range between 0.6 and 1.0 V against a normal hydrogen electrode. Lactone groups could be identified on the oxidized HOPG surface after holding at 1.0 V. Therefore, lactone groups are formed from the reaction between carbonyl groups and carboxylic groups as an intermediate in the electrochemical carbon oxidation reaction to CO2 evolution.