Catalytic NOx reduction system with secondary fuel injection was studied not only in a laboratory scale, but also in a large bench scale. Diesel fuel was injected before the catalyst bed, and the activity and durability of the catalyst for the NOx reduction were tested. Using the fuel as a reductant, silver aluminate supported on alumina was selected as the most active and suitable catalyst. The resistance of the silver aluminate catalyst to SOx was enhanced by the addition of a small amount of WO3, MoO3 and Pt. It was found from the engine bench test that NOx conversion increased in proportion to the amount of aldehydes formed from the injected fuel in the exhaust gas. Thus, the secondary fuel injection into exhaust gas through a partial oxidation catalyst enhanced the NOx conversion. At 450 degrees C, the engine bench test exhibited maximum NOx conversion of 75% and 45% at the SV of 15 000 and 70 000 h(-1), respectively.