The activity of a Ag/Al2O3 catalyst in reducing NOx emissions in a passive mode hydrocarbon selective catalytic reduction (HC-SCR) was investigated using exhaust gas from the diesel engine operation on diesel, biodiesel (RME) and low temperature Fischer-Tropsch synthetic diesel (SD). The HC1 :NOx ratio in the engine exhaust from the combustion of these fuels followed the order: diesel > SD > biodiesel and this order was mirrored in the catalyst activity in reducing NOx in presence of hydrogen (1000 ppm). Compared to diesel fuel, biodiesel combustion produces a higher amount of NOx with reduced concentrations of HCs, while both HC and NOx emissions were reduced in the engine exhaust from the combustion of SD fuel. Although, a higher NOx reduction in the SCR process was seen in the case of diesel fuelling, due to higher HC1 :NOx ratio in the engine exhaust compared to biodiesel (RME) and SD. at low exhaust temperatures (190 degrees C) there was a gradual loss of the catalyst. NOx reduction activity. The incorporation of EGR within the engine operation, increased significantly HC1 :NOx ratios in the exhaust, mainly by lowering the NOx concentration. Under these conditions, which can assumed to represent typical HC1 :NOx ratios of a modern automotive diesel engine, higher NOx conversion was seen with SD, followed by diesel and biodiesel. For all the cases examined here the SD fuelling provides the lowest tailpipe NOx emissions. Hydrogen addition, i.e. at 500, 1000, 1500 and 3000 ppm in the passive mode Ag/Al2O3 SCR catalyst needs to be optimized for the different HC1 :NOx ratios and hydrocarbon species in the exhaust from the combustion of the three fuels. (C) 2010 Elsevier B.V. All rights reserved.