Catalytic activities of various metal oxides for decomposition of nitrous oxide were compared in the presence and absence of methane and oxygen, and the general rule in the effects of the coexisting gases was discussed. The reaction rates of nitrous oxide were well correlated to the heat of formation of metal oxide, i.e., a V-shaped relationship with a minimum at -DeltaH(f)(0) around 450 kJ (O mol)(-1) was observed in N2O decomposition in an inert gas. In the case of metal oxides having the heat of formation lower than 450 kJ (O mol)(-1), CuO, Co3O4, NiO, Fe2O3, SnO2, In2O3, Cr2O3, the activities were strongly affected by the presence of methane and oxygen. On the other hand, the activities of TiO2, Al2O3, La2O3, MgO and CaO were almost independent. The reaction rate of nitrous oxide was significantly enhanced by methane. The promotion effect of methane was attributed to the reduction of nitrous oxide with methane: 4N(2)O + CH4 --> 2N(2) + CO2 + 2H(2)O. The activity was suppressed in the presence of oxygen on the metal oxides having lower heat of formation. On the basis of Langmuir-Hinshelwood mechanism, the effect of oxygen on nitrous oxide decomposition was rationalized with the strength of metal-oxygen bond. (C) 2000 Elsevier Science B.V. All rights reserved.