The structure of gold clusters of different sizes supported on various metal oxides (Al2O3, TiO2, SiO2) exposed to different CO oxidation conditions was investigated in situ using X-ray absorption spectroscopy at the Au L-3 edge. In all catalysts, the only phase detected during catalysis was Au-0. In the most active sample with small gold particles (Au/Al2O3), variation in the electronic structure of the gold clusters with changing reaction conditions was observed by XANES spectroscopy and ascribed to the adsorption of CO on the metallic gold clusters. FEFF8 calculations proved that the changes in the XANES signature of Au/Al2O3 can be explained by backdonation of d-electrons into the pi* orbitals of CO. For Au/Al2O3, the presence of Au-O backscattering in the EXAFS suggested weak cluster-support interactions. For Au/SiO2 and Au/TiO2, the structure of the gold clusters remained unchanged throughout all experiments. (c) 2006 Elsevier Inc. All rights reserved.