Oxidation of methane over yttria/zirconia mixed oxide shows two parallel oxidation pathways, i.e. partial oxidation to CO+H-2+H2O and full oxidation to CO2+2H(2)O. Both reactions proceed via a Mars van Krevelen mechanism with first order kinetics for methane and oxygen and are retarded by water. A mechanism is suggested in which methane is homolitically dissociated over O-(s)(-) sites and via the formation and decomposition of a formaldehyde precursor reacted to CO, H-2 and H2O. In parallel, CO2 is obtained from decomposition of bicarbonate that is formed via further oxidation of dioxymethylene.