The reduction of NO with propene in the presence of excess oxygen and moisture over gold supported on Al2O3 is appreciably enhanced by the addition of Mn2O3 through mechanical mixing. The conversion of NO to Nz is markedly improved at temperatures below 573 K, above which it reaches 98% without appreciable decrease with an increase in temperature. The conversion of NO increases with an increase in O-2 concentration up to 5 vol%, and thereafter it levels off. The role of Mn2O3 is proven to enhance NO2 formation by the reaction of NO with oxygen.