The reaction of H2O2 on SIMFUEL electrodes has been studied electrochemically and under open circuit conditions in 0.1 mol l(-1) NaCl (pH 9.8). The composition of the oxidized UO2 Surface was determined by X-ray photoelectron spectroscopy (XPS). Peroxide reduction was found to be catalyzed by the formation of a mixed U-IV/U-V (UO2+x) surface layer, but to be blocked by the formation of U-VI (UO22+) species on the electrode surface. The formation of this U-VI layer blocks both H2O2 reduction and oxidation, thereby inhibiting the potentially rapid H2O2 decomposition process to H2O and O-2. Decomposition is found to proceed at a rate controlled by desorption or reduction of the adsorbed O-2 species. Reduction of O-2 is coupled to the slow oxidative dissolution of UO2 and formation of a corrosion product deposit of UO3.yH(2)O. (C) 2004 Elsevier Ltd. All rights reserved.