Two types of defect adsorption sites are identified on the ceria support (2097 cm鈭?, 2120 cm鈭?), which are assigned to structural defects and reduced Ce3+ centers. The relative abundance of these sites changes upon thermal treatment. On the Pt nanoparticles, CO first adsorbs on-top at step and edge sites (2066 cm鈭?), before adsorption on the (1 1 1) facets occurs in bridging (1875 cm鈭?) and on-top (2080-2097 cm鈭?) geometry. For the first time, we identify vibrational CO features, which arise from electronic metal-support interactions and from oxygen reverse spillover. On reduced CeO2鈭?/sub>x(1 1 1), a red-shifted band at 2053 cm鈭? appears, which is attributed to increased 蟺-backbonding from Pt to CO, as a result of a change in electronic metal-oxide interaction. Thermal treatment also activates oxygen reverse spillover from the support to the Pt nanoparticle, which gives rise to a blue-shifted feature in the CO spectrum (2090-2105 cm鈭?) due to coadsorbed oxygen next to CO sites.