By means of synchrotron radiation photoemission spectroscopy, we have investigated Pt-CeO 2 mixed oxide films prepared on CeO 2 (111)/Cu(111). Using CO molecules as a probe, we associate the corresponding surface species with specific surface sites. This allows us to identify the changes in the composition and morphology of Pt-CeO 2 mixed oxide films caused by annealing in an ultrahigh vacuum.Specifically, two peaks in C 1s spectra at 289.4 and 291.2 eV, associated with tridentate and bidentate carbonate species, are formed on the nanostructured stoichiometric CeO 2 film. The peak at 290.5-291.0 eV in the C 1s spectra indicates the onset of restructuring, i.e. coarsening, of the Pt-CeO 2 film. This peak is associated with a carbonate species formed near an oxygen vacancy. The onset of cerium oxide reduction is indicated by the peak at 287.8-288.0 eV associated with carbonite species formed near Ce 3+ cations. The development of surface species on the Pt-CeO 2 mixed oxides suggests that restructuring of the films occurs above 300 K irrespective of Pt loadings. We do not find any adsorbed CO species associated with Pt 4+ or Pt 2+ . The onset of Pt 2+ reduction is indicated by the peak at 286.9 eV in the C 1s spectra due to CO adsorption on metallic Pt particles. The thermal stability of Pt 2+ in Pt-CeO 2 mixed oxide depends on Pt loading. We find excellent stability of Pt 2+ for 12% Pt content in the CeO 2 film, whereas at a Pt concentration of 25% in the CeO 2 film, a large fraction of the Pt 2+ is converted into metallic Pt particles above 300 K.