We have examined the interaction between Pt and reduced or oxidized cerium oxide (CeO@sub X@). Reduced CeO@sub X@ strongly modifies the chemisorption of CO on the supported Pt. Two different modifications are evident depending on how the sample is annealed. If the sample is annealed to greater than 800 K, the amount of CO that adsorbs on the Pt at 150 K is dramatically reduced. In addition, annealing at elevated temperatures decreases the Pt XPS intensity significantly. These results suggest that, similar to Pt on reduced TiO@sub X@, the Pt is covered by the reduced CeO@sub X@ at elevated temperatures. If the sample is annealed to 700 K or less before CO is adsorbed at 150 K, there is no decrease in the amount of CO that adsorbs. However, the CO desorption temperature is shifted to a much lower temperature compared to CO adsorbed on a Pt single crystal. This shift in the CO desorption temperature is ascribed to a weakening of the CO - Pt bond that results from an electronic interaction between the Pt and the reduced ceria. The behavior of Pt on CeO@sub X@ is very different from Rh on CeO@sub X@. The CO - Rh bond is strengthened on CeO@sub X@ compared to Rh single crystals and there is an increase in the CO dissociation activity. Rh also shows no evidence of encapsulation when annealed to higher temperatures. CO desorption from Pt on oxidized CeO@sub 2@ resembles that observed from Pt on Al@sub 2@O@sub 3@ and from Pt single crystals and therefore indicates no significant modifications of the Pt chemisorption properties by the CeO@sub 2@. @FootnoteText@ @footnote 1@Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.