Ever since the discovery of heterogeneous gold catalysts for low-temperature oxidation reactions, the mechanism of O2 dissociation on these materials has been controversial. We report Au L3-edge X-ray absorption near-edge structure (XANES) data for an active Au/TiO2 catalyst which indicate that fully reduced metallic gold particles on a reducible support (TiO2) form activated gold-oxygen complexes in the absence of CO. It is possible that these play a role in the mechanism of CO oxidation. These results were obtained with a highly active powder catalyst and the conclusions agree with those derived from model catalysts, for which it was shown that reduced gold is the active component in the oxidation of CO.