Abstract: A method and means for catalytically removing impurities found in the exhaust stream emanating from the internal combustion engine is provided. Such exhaust gases are initially brought into contact with a metallic oxidation catalyst at a temperature sufficient to cause a significant amount of the unreacted oxygen present to be removed therefrom by reacting it with oxidizable materials in the exhaust gases, with the oxidation catalyst employed being characterized by its lack of activity with respect to converting nitrogen and hydrogen found in exhaust gases into ammonia. The so-treated exhaust gases are then brought into contact with a nitrogen oxide (NO.sub.x) reducing catalyst, which is also characterized by its lack of activity with respect to converting nitrogen and hydrogen found in the exhaust gases into ammonia, at a temperature sufficient to cause nitrogen oxide gases in the exhaust gases to be catalytically reduced.