Electrochemical surface oxidation of Vulcan XC-72, a carbon black commonly used in proton exchange membrane (PEM) fuel cells, was studied following potentiostatic treatments up to 120 h at potentials from 0.6 to 1.2 V at room temperature and 65degreesC. Surface oxidation was followed using cyclic voltammetry (CV), thermal gravimetric analysis coupled to on-line mass spectrometry (TGA-MS), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The analytical techniques all indicate significant surface oxidation occurred during the first 16 h of 1.2 V potential holds at room temperature and a slow increase in surface oxide formation thereafter. An identification of ether, carbonyl, and carboxyl surface oxide species was made by deconvolution of XPS spectra and assigning these functional groups to the observed TGA-MS CO2 evolution peaks (150-750degreesC). An increase in CO evolution (>800degreesC) determined by TGA-MS was consistent with electrochemical CV data, which detected electroactive hydroquinone/quinone species; these electrochemically detected species were a minor fraction of the electrochemically generated surface oxides. Potential holds at 1.0 V at room temperature only resulted in slight oxidation of Vulcan XC-72. However, experiments at 65degreesC showed clear signs of surface oxidation after only 16 h at potentials greater than or equal to0.8 V, verifying that surface oxides can be generated under simulated PEM fuel cell conditions. Overall, these results suggest that changes in component hydrophobicity, driven by carbon surface oxidation, are an important factor in determining long-term PEM performance instability and decay. (C) 2004 The Electrochemical Society.