The degradation mechanisms of Pt nanoparticles supported on Carbon have been characterized during oxygen reduction reaction (ORR) conditions using IL-TEM. A TEM grid is used as the sole working electrode allowing a direct correlation between the electrochemical response and the TEM analysis. We mainly observe a decrease in nanoparticle size with some particle disappearance and some particle sintering after potential cycling simulating the start-up and shut-down of a fuel cell. The observation of nanoparticles with reduced particle size provides evidence that dissolution phenomena are the main cause of degradation in Pt/C electrocatalysts, under ORR conditions. © 2011 Elsevier B.V. All rights reserved.