A Au/Ni surface alloy catalyzes the oxidation of CO at low temperature by at least three distinct mechanisms. At the lowest temperature of 70 K, molecularly adsorbed O@sub 2@, spectroscopically characterized by high resolution electron energy loss spectroscopy as peroxo or superoxo species bound at multiple sites with vibrational frequencies of 865 and 950 cm@sup -1@, are the reactants with CO. A third molecularly adsorbed O@sub 2@ species, characterized by an O-O stretch mode at 790 cm@sup -1@, does not react with CO. Between 105-125 K, CO@sub 2@ production coincides with O@sub 2@ dissociation, suggesting a "hot atom" mechanism in which an O atom, formed upon dissociation of adsorbed O@sub 2@, reacts with CO before equilibrating with the surface. The CO that reacts is characterized by a C=O stretch mode at 2170 cm@sup -1@. Given the relatively high frequency, the reacting CO is likely bound to a Au atom. Above 125 K, CO bound to Au reacts with atomically adsorbed O atoms, characterized by a O-Au stretch mode at 660 cm@sup -1@. These results show that nanosize Au clusters bound to oxide supports are not a necessary condition for Au catalyzed, low temperature CO oxidation. In addition, the lower temperature at which the CO oxidation reaction occurs on the Au/Ni surface alloy as compared to the reaction temperature (~200 K) on the supported Au nanoclusters demonstrates that the activation energy is significantly lower on the Au/Ni surface alloy than on Au nanoclusters.