Equilibrium concentrations of dissolved platinum species from a Pt/C electrocatalyst sample in 0.5 M H2SO4 at 80 degrees C were found to increase with applied potential from 0.9 to 1.1 V vs reversible hydrogen electrode. In addition, platinum surface area loss for a short-stack of proton exchange membrane fuel cells (PEMFCs) operated at open-circuit voltage (similar to 0.95 V) was shown to be higher than another operated under load (similar to 0.75 V). Both findings suggest that the formation of soluble platinum species (such as Pt2+) plays an important role in platinum surface loss in PEMFC electrodes. As accelerated platinum surface area loss in the cathode (from 63 to 23 m(2)/g(Pt) in similar to 100 h) was observed upon potential cycling, a cycled membrane electrode assembly (MEA) cathode was examined in detail by incidence angle X-ray diffraction and transmission electron microscopy (TEM) to reveal processes responsible for observed platinum loss. In this study, TEM data and analyses of Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes: (i) Ostwald ripening on carbon at the nanometer scale, which is responsible for platinum particle coarsening from similar to 3 to similar to 6 nm on carbon, and (ii) migration of soluble platinum species in the ionomer phase at the micrometer scale, chemical reduction of these species by crossover H-2 molecules, and precipitation of platinum particles in the cathode ionomer phase, which reduces the weight of platinum on carbon. It was estimated that each process contributed to similar to 50% of the overall platinum area loss of the potential cycled electrode. (c) 2005 The Electrochemical Society.