Gold nanoparticles supported on titania and alumina were used as catalysts for CO oxidation. The oxidation of (CO)-O-16 (2 mol%) with O-18(2) (2 mol%) in He at ambient temperature and 1.2 atm total pressure was performed in a single-pass, fixed-bed reactor. A mixture of CO2 isotopomers was observed during oxidation over Au/TiO2 and Au/Al2O3. However, no oxygen scrambling was observed in and between O-2 and CO. Moreover, evidence for direct exchange between O-2 and CO2 was found. Isotopic transient analysis of a switch between (CO2)-O-16 and (CO2)-O-18 showed that oxygen exchange occurred between the metal-oxide supports and CO2. Therefore, the CO2 isotopomer distribution observed during CO oxidation could rot be attributed solely to the oxidation reaction on An. Comparison of the isotopic transients from C-13 and O-18 studies suggests the primary produc: of (CO)-O-16 oxidation with O-18(2) is (COO)-O-16-O-11. The presence of 0.16 mol % H2O co-fed with the reactants significantly promoted the rate of CO oxidation by factors of 2.5 for Au/TiO2 and 10 for Au/Al2O3. Despite the promotional effect of H2O on the rate of oxidation, no significant differences in the level of oxygen exchange in the CO2 were observed. In addition, the presence of H2O did not result in scrambling of oxygen in O-2-labeled oxygen found in the H2O exiting the reactor appeared to originate from 180 associated with the CO2. (c) 2006 Elsevier Inc. All fights reserved.