Alumina-supported Au nanoparticles were prepared by a deposition-precipitation method using HAuCl4 and a thermal treatment in He. X-ray absorption spectroscopy at the Au L-III edge revealed that the Au particles were predominantly metallic with an average size of 1.2 nm. The kinetics of CO oxidation were measured with various amounts of co-fed H2O in the presence and absence of H-2. Co-feeding H2O significantly increased the rate of CO oxidation, with rates similar to that observed during selective CO oxidation in H-2. The order of reaction with respect to CO decreased from 0.32 to 0.18 by the addition of water vapor. In contrast, the 02 order increased from 0.36 to 0.48 in the presence of H2O. The temperature dependence of the rate changes with water concentration. Although values of the kinetic parameters determined in the presence of H2 were different from those in the presence of co-fed H2O, the general trends were similar. These results suggest that the presence of H2O or H-2 and O-2 have similar promotional effects during CO oxidation on AU/Al2O3.