In polymer electrolyte fuel cells (PEFCs), many kinds of carbonaceous materials are used as electrocatalyst support, gas diffusionlayer, bipolar plate, etc. These carbons are exposed to a strongly oxidative environment of high electrode potentials,high oxygen concentrations, high temperature, and high water content. Therefore, the oxidative degradation of carbonaceousmaterials is a serious issue to be overcome. In this review, we investigate the oxidation of carbonaceous material under conditionsfound in PEFCs, using highly oriented pyrolytic graphite (HOPG) as a model. We also explore a way to mitigate theoxidative degradation of carbon by decorating the surface of HOPG with metal oxides.