Gold clusters on the surface of MgO powder (calcined at 673 K) were prepared from adsorbed [Au(CH3)(2)(acac), where acac is C5H7O2] and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near edge spectroscopy (XANES). One sample initially contained gold predominantly in the form of clusters approximated as Au-6 on the basis of the EXAFS data showing first- and second-shell Au-Au coordination numbers of 4.0 +/- 0.4 and 1.0 +/- 0.1, respectively. The other sample initially contained larger clusters, with an average diameter of about 30 Angstrom (containing about 100 atoms each, on average), as shown by the EXAFS first- and second-shell Au-Au coordination numbers of 9.4 +/- 0.9 and 3.5 +/- 0.4, respectively. The samples, in each of the three gases CO, O-2, and He and in the presence of CO + O-2 during CO oxidation catalysis, were investigated by EXAFS spectroscopy and XANES in a cell that was also a flow reactor. Data obtained during steady-state CO oxidation indicate the presence of gold clusters with an average diameter of about 30 Angstrom, regardless of the initial size of the supported clusters. The XANES results demonstrate the simultaneous presence of both zerovalent and cationic gold in these catalysts.