HC-SCR is more convenient when compared to urea-SCR, since for HC-SCR, diesel fuel can be used as the reductant which is already available onboard the vehicle. However, the DeNOX efficiency for HC-SCR is lower than that of urea-SCR in both low and high temperature windows. In an attempt to improve the DeNOX efficiency of HC-SCR, the effect of hydrogen were evaluated for the fresh and aged catalyst over 2 wt.% Ag/Al₂O₃ using a Euro-4 diesel engine.In this engine bench test, diesel fuel as the reductant was injected directly into the exhaust gas stream and the hydrogen was supplied from a hydrogen bomb. The engine was operated at 2,500 rpm and BMEP 4 bar. The engine-out NOX was around 180 ppm-200 ppm. H₂/NOX and HC₁/NOX ratios were 5, 10, 20, and 3, 6, 9, respectively. The HC-SCR inlet exhaust gas temperatures were around 215°C, 245°C, and 275°C. The catalyst volumes used in this test were 2.5L and 5L for both fresh and aged catalysts.The DeNOX efficiency of the 5L fresh catalyst was in the range of 0-25% without hydrogen, but it increased to 14-79% when the hydrogen was added to the feed stream. The final efficiency was a function of the hydrocarbon and hydrogen concentrations in the exhaust gas mixtures and the HC-SCR inlet temperatures. The NOX conversion efficiency of the 5L aged catalyst, which had been aged thermally at 750°C for 25 hrs in an electrical furnace, was in the range of 6-75% with hydrogen and 2-22% without hydrogen. In case of the 2.5L catalyst, the DeNOX efficiency was in the range of 8-35% for the fresh catalyst and 3-9% for the aged catalyst with hydrogen. In the absence of hydrogen, the NOX conversion rate was in the range of 3-8% and 3-5% for the fresh and aged catalyst, respectively.