Concerns as to the adverse effects of diesel engine exhaust on urban air quality have resulted in increasingly stringent emissions legislation, with the prospect of many major global cities potentially banning diesel vehicles. Emissions of nitrogen oxides (NOx) and particulate matter (PM) are linked to increases in premature mortality, and the simultaneous control of both pollutants through modified combustion strategies presents a significant challenge. In this work, the effects of displacing diesel fuel with hydrogen on exhaust emissions were investigated in both a single cylinder research engine and in a demonstration vehicle. In the initial stage, tests were undertaken on a supercharged, direct injection, single cylinder diesel research engine at different engine loads, intake air pressures and EGR levels. Hydrogen was aspirated with the intake air, and EGR was simulated by supplying the intake pipe with compressed nitrogen gas. The results showed a reduction in CO2 and particulate emissions with increasing H-2 addition, and an increase in NOX emissions at H-2 levels greater than 10% of the total input energy to the engine. The next stage involved tests on a chassis dynamometer with a small van equipped with the multi-cylinder version of the single cylinder research engine. The van was fitted with a programmable H-2 augmentation system, with H-2 addition levels specified by accelerator pedal position. During full drive cycle tests conducted with and without H-2 augmentation up to 10%, an average rate of 1 kW of H-2 was supplied to the engine. With H-2 augmentation, over the total drive-cycle, reductions in CO, NOX and particle number were observed, but a higher total PM mass was recorded. (C) 2018 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.