Catalysts containing 12% Co and 4.5% K, supported on MgO and CeO2 have been studied for diesel soot catalytic combustion. It has been found that this reaction occurs by a redox mechanism when Co and K are deposited on any of the above-mentioned supports. On MgO-supported catalysts, CoOx species are responsible for the supply of oxygen by a redox reaction. In this catalyst, K plays different roles, one of them being the stabilization of the CoOx particles. On CeO2-supported catalysts, Co does not significantly improve the activity of the K/CeO2 catalyst, since in this case the support itself displays redox properties. XPS analyses indicate that the oxygen availability on the surface is much higher on CeO2 than on MgO. On both CeO2 and MgO-supported catalysts, K might provide a route for CO2 release through a carbonate intermediate species. The presence of NO in the gas phase improves the catalytic activity for soot elimination. NO is oxidized to NO2 on the Co, K/CeO2 catalyst, and NO2 is a stronger oxidizing agent than O-2, therefore decreasing the temperature needed to burn the soot.