The addition of Na and S into alumina catalysts brought about a decrease in the catalytic activity for the reduction of NO with ethane in excess oxygen. Aluminas containing Na or S in different amounts were subjected to activity tests for the related reactions to elucidate the causes of the suppressive effects of the addition of Na and S on the reduction of NO. The reactions taken as test reactions were the oxidation of NO with oxygen, the reaction of NO2 with ethene in the absence of oxygen, and the reaction of ethene with oxygen. The addition of Na suppressed the oxidation of NO, and the reaction of NO2 with ethene to form N-2, but promoted the reaction of ethene with oxygen to a great extent. The addition of Na also caused the formation of NO in the reaction of NO2 with ethene. The changes which the addition of Na brought about are all unfavorable directions for the reduction of NO. The most important effect of the addition of Na on the decrease in the reduction of NO is suggested to be due to the enhancement of the reaction of ethene with oxygen. The addition of S suppressed the oxidation of NO to a great extent, but did not affect much the reaction of ethene with oxygen. Like the case of the Na addition, the addition of S caused the formation of NO in the reaction of NO2 with ethene.