The influence of gas-phase oxygen on platinum surface oxidation in polymer electrolyte membrane fuel cell (PEMFC) cathodes was investigated. Catalyst layers were exposed to 4, 21, and 100% O-2 for a fixed time at +0.85 and +0.95 V-RHE. The extent of surface oxidation was characterized using cyclic voltammetry (CV). The extent of Pt surface oxidation increased relative to the corresponding O-2-free potential hold at +0.85 V-RHE. The extent of oxidation was directly proportional to oxygen concentration, and increased with time exposed to the oxygen-containing gas. Pt surface oxidation was not influenced by gas-phase O-2 concentration at +0.95 V-RHE. (C) 2004 The Electrochemical Society.