The suppression of the CO oxidation activity by SO, treatment in Au/TiO2 catalyst was investigated by means of reaction experiments, in situ IR and XPS. The CO oxidation activity over Au/TiO2 catalyst decreased after SO2 treatment. The oxidation state of An particles did not change after SO2 treatment. The XP spectra in the S 2p region showed that sulfate was formed on TiO2 supports after SO2 treatment. The IR experiment revealed that SO2 treatment increased the adsorption strength between Au and CO. Carbonate peak was absent for the SO2-pretreated sample in the presence of CO. The migration of CO was completely blocked by surface sulfate compound. Strong and irreversible formation of sulfate at the perimeter is the main reason of the loss of CO oxidation activity at low temperature CO oxidation. (c) 2005 Elsevier B.V. All rights reserved.