One of the processes responsible for performance degradation of a polymer electrolyte fuel cell (PEFC) is the loss of the electrochemically active surface area of the platinum-based electrocatalysts, due in part to platinum dissolution. The long-term dissolution behavior of polycrystalline platinum and high-surface-area carbon-supported platinum particles was studied under potentiostatic conditions relevant to PEFC cathode conditions. The equilibrium concentration of dissolved Pt was found to increase monotonically from 0.65 to 1.1V (vs SHE) and decrease at potentials > 1.1 V. Dissolution rates measured at 0.9 V were comparable for the two types of electrodes (1.4 and 1.7 x 10(-14) g/cm(2) s). (c) 2006 The Electrochemical Society.