The corrosive operating conditions of a PEM fuel cell causes significant oxidation of its carbon supports, thus severely affecting the fuel cell performance and lifetime. PEM fuel cells in transportation or automobile applications typically experience potential cycling due to start-up/shutdown cycles or varying loads further deteriorating the long-term performance of a fuel cell. Here we report that the rate of surface oxidation of carbon supports significantly increases during potential cycling making the carbon support prone to further oxidation. Using X-ray photoelectron spectroscopy, we identify the various carbon-oxygen groups formed on the surface of carbon support due to potential cycling and compare them with those treated under potential hold conditions. Interestingly, these surface groups vary in proportion from those formed on carbon support during idle potential conditions reported in the literature. (C) 2010 Elsevier Ltd. All rights reserved.