The nature of platinum dissolution and precipitation in a polymer electrolyte membrane of a membrane electrode assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using a potential holding experiment at 1.0 V vs a reversible hydrogen electrode and high-resolution transmission electron microscopy. The electrochemically active surface area decreased depending on the holding time, and platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. However, platinum dissolution and deposition out of the catalyst layer were greatly reduced when a platinum black electrode was used. In the experiment using a double-layered catalyst layer, platinum redeposited not on the carbon black surface but rather on the platinum black surface. (c) 2006 The Electrochemical Society.