Gold supported on alumina catalysts were investigated for selective NO x reduction by propene. Two types of alumina, commercial and sol-gel alumina, were employed. The catalytic performance was mainly studied over the impregnated sol-gel alumina catalyst. The catalytic activity was tested under various propene and oxygen concentrations. NO x conversion and the activity window of the catalysts increased as the propene or oxygen concentration increased. The presence of water also enhanced the NO x conversion and widened the activity window of the catalysts. The sol-gel alumina supported catalysts gave much higher activity than the commercial alumina supported catalysts.