A mechanism of catalyst particle growth in proton exchange membrane fuel cells (PEMFCs) by Ostwald ripening is presented. Particle growth occurs as a coupled process involving platinum ion transport through aqueous liquid and/or ionomer, and electron transport through a carbon support. The dominating factor in degradation of a catalyst supported on carbon is the presence of platinum ions in solution (in liquid and/or in ionomer). Experiments were conducted on commercial PEMFC electrodes in three liquids, PtCl4 solution, dilute acid solution, and deionized water. Pt particle size grew from similar to 4 nm to >20 nm after 1 week in PtCl4 solution. By contrast, no detectable growth occurred in dilute acid or in deionized water. This demonstrates that the higher the Pt ion concentration, the faster the kinetics. The role of electronic conduction through support was verified by conducting experiments in PtCl4 solution on Pt supported on an electronically insulating material, namely alumina. While significant growth occurred in Pt supported on carbon, no detectable growth occurred in Pt supported on alumina. This observation is in complete accord with the model, and demonstrates the role of electronic transport on degradation. That is, when supported on alumina, lack of an electronically conducting path suppresses degradation even with PtCl4 present in solution. (c) 2007 The Electrochemical Society.