The physical and chemical state of proton exchange membrane fuel cell (PEMFC) electrocatalysts (Pt/C) were investigated after 529 h of operation under fuel cell relevant conditions (333 K, 0.12 W cm(-2)) and 123 h of rest time under inert atmosphere (N-2). Upon aging, pronounced corrosion of the cathode electrocatalyst (carbon-supported platinum nanoparticles: Pt/C) was evidenced by field-emission gun scanning electron microscopy (FEG-SEM), high-resolution transmission electron microscopy (HRTEM), and electrochemical techniques. Carbon corrosion was witnessed by the decrease of the cathode thickness (-60%) and by the presence of nonsupported Pt particles inside the electrode. At the cathode, the corrosion of Pt nanoparticles produces Pt-(*)(z+) ions which diffuse in the ionomer phase or in solution [(*) stands for ionic species present in the ionomer phase or in solution]. These ions are highly mobile inside the membrane electrode assembly (MEA) and may cross over from the cathode to the anode through the PEM. The driving force for that is the electro-osmotic drag (if Pt-(*)(z+) ions combine with anions and carry a negative charge) and the chemical diffusion (concentration gradient of oxidized platinum species). Consequently, Pt-(*)(z+) ions were detected by ultraviolet (UV) spectroscopy in the PEM. Due to the high mobility of Pt-(*)(z+) ions, FEG-SEM and HRTEM analysis of the cross sections of MEAs revealed a pronounced change of Pt distribution after operation. Size distributions of both anode and cathode electrocatalysts evidenced an increase of the mean particle size, tailing toward large particle sizes and particle agglomeration. Nonspherical Pt nanoparticles were detected inside the PEM, the size and shape distribution of which strongly depend on the distance from the cathode. We believe Pt-(*)(z+) ions are reduced chemically inside the membrane by H-2 crossing over the PEM and both chemically and electrochemically at the anode/cathode. (c) 2006 The Electrochemical Society.