The interaction of oxygen with catalytic surfaces of MgO, Sm2O3, and Sm2O3 (5 mol%)/MgO and the reaction of methane with short-lived surface oxygen species over Sm2O3 were studied by transient experiments. The oxygen exchange between gas-phase O-18(2) surface oxygen O-16(s) was investigated in the temperature range from 539 to 1035 K. The oxygen-exchange activity decreased from Sm2O3 (E(a) = 81 kJ/mol) via Sm2O3/MgO (E(a) = 96 kJ/mol) to MgO (E(a) = 261 kJ/mol). The presence of methane did not influence the exchange rate of O-18(2), with the surface. This indicates (a) that oxygen activation on the surface is faster than activation of methane and (b) that neither gas-phase oxygen nor nondissociated surface oxygen is involved in the reaction with methane under the reaction conditions applied. It was shown that the surface reaction of methane with oxygen intermediates formed from gas-phase O-2 is faster than the desorption of any diatomic oxygen species. Over Sm2O3, the degree of oxidative conversion of methane and the resulting product distribution are determined by the amount of surface-oxygen species formed by dissociation of gaseous O-2 on the catalyst surface. It is suggested that these active oxygen sites consist of highly reactive short-lived species (below 200 ms) and also species having longer lifetimes.