Barium oxide supported on magnesium oxide (Ba/MgO) exhibits unusual behavior as a catalyst for NO decomposition at Ba loadings of 11 mol % or greater. The catalytic activity is characterized by a sharp decrease in activity when the reaction temperature exceeds a certain value that depends on the partial pressures of NO and O-2, The fall-off temperatures are between 630 and 700 degrees C for concentrations between 1% and 4% NO in helium. At 700 degrees C with 4% NO, the N-2 formation rate was 2.5 mu mol g(-1) s(-1) for a 14 mol % Ba/MgO catalyst. The discontinuity in activity is accompanied by an abrupt change in activation energy for the reaction, which suggests a change in mechanism. In situ Raman spectroscopy was used to follow temporal changes in the composition of the catalyst that occurred after step changes in temperature or NO concentration. A comparison of spectroscopic and catalytic results indicates that a barium-nitro phase is an intermediate in the catalytic cycle below the fall-off temperature. Nitro species at the surface apparently react with NO to form the decomposition products, N-2 and O-2. This is believed to be the rate-limiting step in the catalytic cycle. Peroxide ions may be involved in the formation of the nitro intermediate.