Hydrocarbon-selective catalytic reduction (HC-SCR) is a deNO(x) system for diesel engines, which uses onboard fuel as the reductant to simplify the system. However, HC-SCR is relatively inefficient at reducing NOx, especially at low temperatures. Here we investigate improvements in the efficiency resulting from the addition of hydrogen. We investigated the effects of adding hydrogen on the efficiency of NOx reduction via HC-SCR using various reductants. Tests were carried out using both laboratory experiments and an engine test-bench at temperatures below 315 degrees C, with a 2.5 wt.% high-dispersion Ag/Al2O3 catalyst. The hydrogen was introduced to maximize the efficiency of NOx reduction at low temperatures (245-315 degrees C). In the laboratory tests, propene, heptane, and dodecane were used as reductants to determine the effects of different chain lengths and chemical structures. The effects of hydrogen addition on the gas-phase reaction of HC-SCR were also examined. The efficiency of NOx reduction increased significantly following the addition of hydrogen. In particular, the effects of hydrogen addition were significantly influenced by characteristics of the fuel, including the chain length and the structure, and longer chains and a higher degree of saturation were found to be advantageous. Engine tests were carried out to verify the effects of longer hydrocarbons in the presence of hydrogen; we found a maximum NOx reduction efficiency of 79% at 315 degrees C and 74% at 245 degrees C using diesel fuel as a reductant. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.