The co-combustion of diesel fuel with H-2 presents a promising route to reduce the adverse effects of diesel engine exhaust pollutants on the environment and human health. This paper presents the results of H-2-diesel co-combustion experiments carried out on two different research facilities, a light duty and a heavy duty diesel engine. For both engines, H-2 was supplied to the engine intake manifold and aspirated with the intake air. H-2 concentrations of up to 20% vol/vol and 8% vol/vol were tested in the light duty and heavy duty engines respectively. Exhaust gas circulation (EGR) was also utilised for some of the tests to control exhaust NOx emissions. The results showed NOx emissions increase with increasing H-2 in the case of the light duty engine, however, in contrast, for the heavy duty engine NOx emissions were stable/ reduced slightly with H-2, attributable to lower in-cylinder gas temperatures during diffusion-controlled combustion. CO and particulate emissions were observed to reduce as the intake H-2 was increased. For the light duty, H-2 was observed to auto-ignite intermittently before diesel fuel injection had started, when the intake H-2 concentration was 20% vol/vol. A similar effect was observed in the heavy duty engine at just over 8% H-2 concentration. (C) 2018 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY license(http://creativecommons.org/license/by/4.0/).