The catalytic combustion of methane over ceria, lanthana, and the entire range of mixed oxides has been studied under reaction conditions free from limitations set by the transfer of heat and mass. Bulk structures of the catalysts were determined by X-ray diffraction and their surface compositions were monitored by XPS. Their surface areas were measured by BET methodology. Surface basicity was measured by temperature-programmed desorption of carbon dioxide. The combustion rate was close to first order on methane partial pressure, close to zeroeth order on oxygen partial pressure, and strongly retarded by carbon dioxide. A particular study was made of the solid solutions of lanthana in ceria where, operated at temperatures just above that of threshold activity, the two metal oxides act synergistically in the catalytic combustion of methane. It is proposed that cerium and lanthanum interact to form the catalytic active centre providing both redox and base functions. (C) 2003 Published by Elsevier Inc.