The structural evolution of two Pt/ceria-zirconia catalysts, characterized by different amounts of supported Pt, was monitored by in situ X-ray diffraction during the anaerobic oxidation of CO at different temperatures. In a first phase, oxygen coming from the surface layers of the ceria-zirconia mixed oxide is consumed and no structural variation of the support is observed. After this induction time, bulk reduction of Pt/ceria-zirconia takes place as a step-like process, while the CO 2 production continues at a nearly constant rate. This behavior is totally different from that of the metal-free support in similar reaction conditions, that show a gradual bulk reduction. In repeated oxidation-reduction cycles, it was observed that the induction time in Pt/ceria-zirconia is a function of the thermal history, of the amount of supported Pt and of the structural evolution of the samples.