In situ FT-IR measurements for Au/TiO2 and Au/Al2O3 have been carried out under the flow condition of CO oxidation at atmospheric pressure. It has been found that the Au particles remain neutral (Au) in the presence of oxygen, while negatively charged particles (Au delta-) is formed in the absence of oxygen, as a result of the charge transfer from the oxygen vacancies. Moisture did not significantly affect the adsorption states of CO over Au/TiO2 and Au/Al2O3. Enhancement of the CO2 production by moisture was observed over Au/A1203, which is accompanied by the decomposition of carbonate-like species by moisture. (C) 2007 Elsevier B.V. All rights reserved.