The present study aims to investigate the electrochemical reduction of a range of H2O2 concentrations on a 1.5 at.% SIMFUEL rotating disk electrode over the pH range 1-4. The peroxide reduction mechanism is determined to occur either on a U-V-containing surface layer of composition (U1-2xU2xO2+x)-U-IV-O-V or on an adsorbed U-V-containing surface intermediate depending on the surface composition which is determined by solution pH and H2O2 concentration. The (U1-2xU2xO2+x)-U-IV-O-V catalytic surface lattice layer, if formed, is stable and rotation disk studies have demonstrated that H2O2 reduction on this surface achieves the diffusion-controlled limit at sufficiently negative overpotentials. However, the adsorbed U-V-containing surface intermediate is unstable and can be destroyed by electrochemical reduction to its original state, i.e. UO2, or by chemical oxidation to U-VI prior to dissolution as UO22+. The instability of this surface intermediate limits its availability which prevents significant H2O2 reduction and yields currents below the diffusion-controlled limit. The occurrence of both reduction mechanisms demonstrates the influence of locally established surface compositions and the switch from one to the other appears to be controlled by surface diffusion conditions and the bulk pH and H2O2 concentrations. (C) 2012 Elsevier Ltd. All rights reserved.