The catalytic oxidation of CO at 300 K over a Au/Al2O3/NiAl(110) surface was studied. No CO2 formed in the absence of water in the CO/O-2 reaction gas mixture, whereas CO was oxidized when water was present; the CO oxidation rate increased with increasing water partial pressure and with decreasing particle size of Au. The water was shown to promote CO oxidation over the Au/Al2O3/NiAl(110) surface, as has been previously observed for a Au/TiO2(110) surface. We demonstrated that the active sites for and role of water in CO oxidation over Au/Al2O3/NiAl(110) were identical to those over Au/TiO2(110). However, more water was required to increase the CO oxidation rate over Au/Al2O3/NiAl(110), compared to that required over Au/TiO2(110). On the basis of temperature-programmed desorption measurements of adsorbed water on the support surfaces, we concluded that the difference in the dependence of CO oxidation on between Au/Al2O3/NiAl(110) and Au/TiO2(110) could be attributed to the difference in the affinity for water between the Al2O3/NiAl(110) and TiO2(110) supports.