The catalytic combustion of carbon monoxide over ceria, lanthana, and their mixed oxides has been studied under reaction conditions free from heat and mass transfer limitations. The bulk structures, surface compositions, and surface areas of the catalysts were determined by XRD. XPS. and BET methods, respectively. Doping either ceria or lanthana with low levels of the other oxide retards the reaction. The reaction order on carbon monoxide is almost first-order over ceria, but over the lanthana-containing catalysts it changes from positive to negative with increasing partial pressure. With all catalysts, the rate tends to zeroth order on oxygen with increasing partial pressure. An extended study was made of the major solid solution, lanthana-in-ceria, which operated near-threshold temperatures is poisoned by lanthana. The active centres are ensembles of cerium atoms, the size being lowered by lanthanum. The retardation is discussed in terms of interactions between cerium ensembles and anionic vacancies. (C) 2003 Published by Elsevier Inc.