A thorough comparison of gold catalysts on different support materials as well as activity measurements for Au on mixed oxides (Au/Fe2O3. MgO) reveal enhanced CO oxidation rates for a group of "active" support materials (Fe2O3, TiO2, NiOx, CoOx), For Au/Fe2O3, it is shown that large amounts of oxygen can adsorb on the support, which most likely represents the oxygen supply during reaction. The high mobility of these oxygen species and the absence of oxygen scrambling with labeled O-36(2) in pulse experiments strongly suggest the adsorption in a molecular form on the iron oxide support. From the absence of the doubly marked product (COO)-O-18-O-18, reaction schemes via a carbonate-like intermediate or transition-state can be ruled out. For Au catalysts supported on active materials, the dominant reaction pathway is concluded to involve adsorption of a mobile, molecular oxygen species on the support, dissociation at the interface, and reaction on the gold particles and/or at the interface with CO adsorbed on the gold. The facile supply with reactive oxygen, via the support, serves as a probable explanation for the observed independence of the turnover frequency from the Au particle size on these catalysts, while for Au supported on inert materials, where the oxygen supply most likely proceeds via direct dissociative adsorption on the Au particles, the size of the latter plays a decisive role. (C) 2001 Academic Press.