Gold nanoparticles supported on hydrous tin oxide (Au-SnOx) and mixed with Vulcan carbon are active electrocatalysts for the four-electron oxygen reduction reaction (ORR) to water in acid electrolyte below 0.55 V vs a reversible hydrogen electrode. In comparison, gold on Vulcan carbon (Au/VC) is inactive for the ORR in acid and generates mostly peroxide (i.e., two-electron reduction). These results imply that the SnOx support imparts the ORR activity of the Au. We study this metal support interaction by in situ X-ray absorption fine structure and near edge spectroscopy at the Au L-3 and Sn K edges in an electrochemical cell. The results for 10 wt % (2 nm) and 20 wt % (2.7 nm) Au nanoparticles supported on SnOx suggest that a bifunctional mechanism plays the dominant role in the ORR reaction below 0.55 V. O-2 adsorbs and dissociates on the SnOx surface with apparent simultaneous electron transfer from the Au and then is reduced further to water by electron transfer from the Au. Enhanced O[H] adsorption occurs at potentials above 1.2 V on the Au-SnOx catalysts. This O[H] adsorption is enhanced through a ligand mechanism involving the SnOx support but has no apparent effect on the ORR. (C) 2008 The Electrochemical Society.