CO adsorption and oxidation over Au/Al2O3 were studied by infrared (IR) spectroscopy at 150 K. Two types of CO adsorption were found on the support, Al2O3, With the existence of gaseous CO (266 Pa). An additional band of adsorbed CO at 2110 cm(-1) was found on Au/Al2O3 at 150 K in the presence of gaseous CO and was assigned to CO adsorbed on ultrafine gold particles dispersed on Al2O3 support After evacuation at the same temperature, the band of CO adsorbed on the, ultrafine gold particles decreased in intensity and shifted to 2118 cm(-1), whereas the CO adsorbed on Al2O3 disappeared. The band at 2118 cm(-1) was assigned to CO irreversibly adsorbed on the ultrafine gold particles at 150 K. CO oxidation by O-2 took place on Au/Al2O3 in the presence of CO and O-2 gas mixture even at 150 K, whereas CO irreversibility adsorbed on Au/Al2O3 and could not be oxidized by the exposure to O-2 at the same temperature. Therefore, the reversibly adsorbed CO on the ultrafine gold particles was found to be the active CO species for CO oxidation at 150 K. By spectral analysis, the reversibly adsorbed CO was found to show a band at 2108 cm-l. The ultrafine gold particles were oxidized by O-2 treatment at 573 K, becoming inert for CO oxidation at 150 K, but were easily rereduced at 573 K by evacuation treatment. The recovered Au/Al2O3 also catalyzed CO oxidation at 150 K.