Density functional theory calculations for the CeO2 (111) surface doped with An, Ag, and Cu show that the bond between the oxygen atoms and the oxide is weakened by presence of the dopant. In CO oxidation, doping of CeO2 with An allows the oxide to react readily with CO and make carbonates. These decompose to release CO2 and form an oxygen vacancy on the surface. The vacancy adsorbs oxygen from the gas and weakens its bond, so that it reacts with CO to form a carbonate, which decomposes to release CO2 and heal the oxygen vacancy. To be a good oxidation catalyst, a doped oxide must achieve a balance between two conflicting requirements: It must make surface oxygen reactive but not so much that it will hinder the healing of the oxygen vacancies created by the oxidation reaction. (c) 2006 Elsevier Inc. All rights reserved.