The reduction of NO with propene in the presence of excess oxygen and moisture over gold supported on Al 2 O 3 is appreciably enhanced by the addition of Mn 2 O 3 through mechanical mixing. The conversion of NO to N 2 is markedly improved at temperatures below 573K, 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 5vol%, and thereafter it levels off. The role of Mn 2 O 3 is proven to enhance NO 2 formation by the reaction of NO with oxygen.