Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH3)(2)(C5H7O2) and characterized in reactive atmospheres by X-ray absorption and infrared spectroscopies. Under conditions of CO oxidation catalysis at 298 K and 760 Torr in a flow reactor, the gold remained as mononuclear species, as demonstrated by extended X-ray absorption fine structure spectra indicating the lack of Au-Au contributions. X-ray absorption near edge structure data and infrared spectra in the v(co) region indicate that the initially present Au-111 was reduced to Au-1 as the catalyst lost activity during operation in the flow reactor, and the data give no evidence of the formation of clusters of zerovalent gold. The Au-111 complex is an order of magnitude more active as a catalyst than the Au-1 complex. These results are the first evidence of supported mononuclear cationic gold complexes as CO oxidation catalysts.