The distribution patterns of Pt crystals that have moved from electrodes to the membrane phase of membrane electrode assembly (MEA) are monitored using transmission electron microscopy (TEM) after long-term operation (> 1000 h) of a polymer electrolyte membrane fuel cell (PEMFC) at various operating and feed conditions. The dissolution of cathode Pt and subsequent migration to the membrane is readily observed when residual oxygen concentrations inside the cathode are kept high under low current density conditions. Dissolution of anodic Pt can also be observed under constant-current operation when hydrogen feed is kept low to induce a hydrogen shortage on the Pt surface. It is postulated that the Pt at the both electrodes is dissolved by chemical oxidation to PtO in the presence of residual oxygen. The Pt ions that are dissolved in water migrate to the membrane phase and undergo repeated oxidation/dissolution and reduction/deposition by crossover of oxygen and hydrogen, respectively. As a result, the distribution patterns and Crystal sizes of the migrated Pt are strongly dependent on the relative concentrations of the crossover oxygen and hydrogen. The final position of the deposited Pt band is located at the point where crossover oxygen becomes depleted, typically between 1 and 10 mu m from the cathode-membrane interface. Higher concentrations of oxygen and hydrogen in the membrane yield sharper and narrower Pt bands with large Pt aggregates. whereas lower concentrations yield wider distribution bands with smaller Pt crystals. (C) 2008 Elsevier B.V. All rights reserved.