Effects of adding CeO 2 to platinum on alumina were studied by XPS, XRD, and TPR. Whereas heating of Pt on alumina in air at 800 °C results in a surface interactive Pt species having a chemical state of Pt +4, attributed to PtO 2, the same treatment of Pt on CeO 2 or on CeO 2 Al 2O 3 yields a platinum-ceria interactive species with an oxidation state of Pt +2, attributed to PtO. When heated in H 2 at 500 °C, Pt on both CeO 2 and CeO 2 Al 2O 3 is completely reduced to Pt 0. Upon reduction in H 2 at 920 °C, Pt assists in the formation of CeAlO 3. In the reverse reaction, Pt facilitates bulk oxidation of CeAlO 3 to CeO 2 at 500 °C.