The initial NOx reduction activity of several alumina-supported, platinum-group metal catalysts is evaluated in vehicle tests. The experimental vehicle was equipped with two 36 in3 NOx converters, and a 260 in3 oxidation converter containing a pelleted platinum catalyst, rich carburetion, and exhaust gas recirculation.The NOx catalysts included Ru, Ru-Pt, Ru-Pd, mechanical mixtures of Ru and Pt, Pt-Ni, Pt, and Pd. As evaluated over the 1975 FTP, the NOx reducing effectiveness of these catalysts was: Ru combinations > Pt-Ni > Pt ≈ Pd.Modulated air injection was used, enabling the NOx converters to be used as oxidizing converters during vehicle start-up. Operation in this mode greatly reduced HC and CO emissions, with an acceptable increase in NOx emissions.In addition, the NOx reduction performance of all the NOx catalysts was enhanced by bleeding a small amount of air into the NOx converters. Minimum NOx emissions corresponded to air bleed rates of 3-4% of the total engine airflow. The air bleed effect was due to decreased NH3 formation over Ru, and to increased NOx activity due to higher catalyst temperatures over Pt.