Infrared spectroscopy of CO adsorption at low temperature has shown the presence of oxidized gold species (Au+ and Au3+) on nanocrystalline Au/CeO2 catalyst. This species are quite stable even under reduction conditions like the CO oxidation reaction. The concentration of this species decrease by increasing the pre-treatment temperature of the AU/CeO2 catalyst which is in accordance with a decrease in the catalytic activity of the sample. In accordance oxidized gold species are involved in the catalytic activity of Au/CeO2 catalyst for CO oxidation although the participation of metallic sites cannot be excluded. (c) 2006 Elsevier B.V. All rights reserved.