The electrooxidation of carbon black, which contains Pt electrocatalyst particles, was investigated in concentrated phosphoric acid at 0.6 to 1.0V. At the high potentials, anodic dissolution of Pt is rapid, and consequently no metal is present to catalyze the corrosion of carbon at 160 °C in 98% H3PO4. On the other hand, at 0.6V anodic dissolution of Pt is negligible, and hence it is present to catalyze the corrosion of carbon. In fact, the measurements indicate that the corrosion rate is noticeable higher than that of carbon black without Pt. These results suggest the Pt particles with surface Pt-0 may serve as an intermediary which facilitates the corrosion of carbon.