The oxidation of a mixture of four hydrocarbons and carbon monoxide has been studied over alumina-supported palladium, platinum and rhodium catalysts made using chloride precursor salts. The order of the light-off temperatures for the hydrocarbons is the same for each metal (l-hexene, toluene, benzene and finally iso-octane) but the separation between each and their occurrence relative to carbon monoxide varies. The absolute values, especially for iso-octane, are strongly dependent on the pretreatment procedure. Exposure to H-2/H2O in the place of O-2 gives much higher activity. Activation can also be achieved by repeated runs with the reactant mixture. Chlorobenzene is produced in small amounts during experiments using partially activated catalysts especially with rhodium. Other reasons for believing that the development of activity is at least partly associated with the removal of chlorine are discussed.