The reduction of NO over silver/alumina catalysts was studied in a flow reactor system at W/F (catalyst weight/total flow)=0.024-0.012 g/(ml/s). Silver/alumina was shown to catalyze the reduction of NO by propylene or propylene/propane mixture in excess oxygen. The catalytic activity was attributed to the hydrocarbon-oxidation reactivity of a silver catalyst. The activity for NO-reduction increased as the oxygen-content in reaction mixture increased from 1.5% to 10%. This effect of oxygen-content and the temperature range around 500 degrees C for the reduction of NO can be correlated to the activity of hydrocarbon-oxidation. The presence of water and sulfur dioxide had adverse effect on the reduction of NO. The adverse effect could be partially compensated by the enhancement of activity using a proper type of alumina. The type of alumina used had a great effect on the activity of NO-reduction for a silver/alumina catalyst. Preliminary characterization showed that the average pore size and the pore-size distribution of the alumina could be important factors. An active silver/alumina catalyst was prepared with a higher NO-conversion than the best Cu/ZSM5-based catalyst evaluated at Ford under the same reaction conditions in the presence of H2O and SO2.