The electrochemical reduction of oxygen was studied on Au/C catalysts (20 and 30 wt%) in 0.5 M H2SO4 and 0.1 M KOH solutions using the rotating disk electrode (RDE) method. The thickness of the Au/C-Nafion layers was varied between 1.5 and 10 mu m. The specific activity of Au was independent of catalyst loading in both solutions, indicating that the transport of reactants through the catalyst layer does not limit the process of oxygen reduction under these conditions. The mass activity of 20 wt%Au/C catalysts was higher due to smaller particle size. The number of electrons involved in the reaction and the Tafel slopes were found; the values of these parameters are similar to that of bulk polycrystalline gold and indicate that the mechanism of O-2 reduction is not affected by carbon support or the catalyst configuration. (C) 2009 Elsevier Ltd. All rights reserved.