The reduction of oxygen has been studied on nanostructured gold electrodes in 0.5 M H2SO4 and 0.1 M KOH using the rotating disk electrode (RDE) technique. Thin films of gold, with a nominal thickness of 0.25-20 nm, were prepared by vacuum evaporation onto glassy carbon (GC) electrodes and coated with a Nafion film. The surface morphology of the Au films was examined by transmission electron microscopy (TEM) and their structure by glancing incidence angle X-ray powder diffraction (GIXRD). Tafel slopes close to -120 mV dec(-1) were obtained for all electrodes in both solutions, indicating that the O-2 reduction mechanism is the same for thin-film as for bulk Au electrodes. The specific O-2 reduction activity (SA) of the nanostructured Au electrodes in 0.5 M H2SO4 was almost constant over the range of Au thicknesses studied. In 0.1 M KOH, the SA slightly decreased with decreasing film thickness. (c) 2007 Elsevier B.V. All rights reserved.