Barium oxide supported on magnesium oxide (Ba/MgO) is an effective catalyst at elevated temperatures for the selective catalytic reduction (SCR) of nitric oxide, with methane as the reductant. The rate of the SCR reaction is greater than either the rate of reduction with CH4, in the absence of O-2, or the rate of decomposition of NO. At 650 degrees C the rate of the SCR reaction over 1 mol% Ba/MgO was 0.19 mu mol g(-1) s(-1) with 1% NO, 0.25% CH4, and 0.5% O-2. The SCR reaction is believed to occur via gas-phase CH3 radicals that are formed at the surface. First-order kinetics with respect to CH4 is consistent with radical formation being the rate-determining step. The activities of the catalysts and the overall reaction order are influenced by CO2, which is a product of the reaction, but also would be present in an exhaust stream. The inhibiting effect of CO2 is much more significant for a catalyst containing 4 mol% Ba than for one containing 1 mol% Ba. In the latter case, the MgO support modifies the basicity of the BaO. In the absence of O-2, CO also is a reductant for NO, but if O-2 is present, the nonselective oxidation of CO occurs. That is, CO reacts preferentially with O-2 instead of NO.