Gold catalysts supported on gamma-Al2O3 (Au/gamma-Al2O3) were prepared by deposition-precipitation of aqueous HAuCl4 and, alternatively, by deposition of Au(CH3)(2)(acac) from pentane solution. The samples were characterized by in situ XANES and temperature-programmed reduction and by their performance as CO oxidation catalysts at room temperature. The Au was found to be present as Au(III) in both as-prepared catalysts, and the Au(III) was stable upon exposure to reducing gases at room temperature. Reduction of Au(III) occurred at elevated temperatures, and the rate and extent of reduction were found to depend strongly on the reducing conditions. Water vapor facilitated reduction, but only after the sample had been reduced to some extent by CO or H-2. Exposure of an as-prepared catalyst to a catalytically reacting mixture of CO + O-2 at 100 degreesC was effective in activating it, though to a lesser extent. The data indicate that zerovalent Au is necessary for catalytic activity, but there is no correlation between the activity and the extent of reduction, implying that cationic Au may play a role in the catalytic sites. Water or surface species derived from water also appears to play a significant role.