A catalyst for the oxidation of carbonaceous particulates carried in a gaseous phase, for example for the treatment of Diesel engine exhaust emissions, comprises a substrate for permitting flow of the gaseous phase therethrough coated with a high surface area ceramic oxide (for example alumina made by a vapor phase condensation method such as flame hydrolysis), cerium(IV) oxide, and a compound such as an oxide, carbonate or nitrate of an element of Group 1A of the Periodic Table. The substrate may be in the form of a ceramic honeycomb such as of cordierite, or of metal sheets (e.g. of an aluminium bearing ferritic alloy) shaped and wound to define gas flow channels, or of ceramic or metal foams, particles or wires in random or structured arrangement. The catalyst may include other components (e.g. transition metals such as Co, Cu, and Mn; Pb; and precious metals such as Pt and Pd) for enhancing performance.