Three Au/TiO2 catalysts, with the same Au loading and with different particle sizes, were prepared by the deposition-precipitation method followed by calcination at three different temperatures, 473, 573, and 873 K. The mean diameters of Au particles were 2.4,2.5, and 10.6 nm, respectively. On all the samples the CO adsorption and different CO-O-2 interactions were examined by FTIR at 90 K and at room temperature. The higher catalytic activity on CO oxidation found for the samples calcined at 473 and 573 K is related to the higher concentration of step sites over the Au surfaces and to a higher concentration of step sites at the borderline with the support. At 90 K, CO and molecular oxygen are competitively adsorbed on step sites. By CO preadsorption on hydrated catalysts, the reaction with O-2 gives CO2 already at 90 K, while by oxygen preadsorption the reaction is completely inhibited, unless moisture is present in the gas phase. An effect of CO coadsorption has been evidenced on water dissociation on gold sites or at the interface with the support, producing atomic hydrogen. The hydrogen reacts with the oxygen, producing a reactive species, quickly dissociated in nascent oxygen and OH groups. Moreover, the reaction with O-18(2) at 90 K in the presence of moisture produces only (COO)-O-16-O-18, giving evidence that there is no direct participation of oxygens of the support and of the water in the reaction. At room temperature, other reaction channels become operative, involving oxygen species activated on the support, as shown by the extensive exchange reactions occurring with the support oxygen.