Concerns over the impact of road transport emissions on the climate have led to increased focus on how CO2 emissions could be reduced from the sector. This is of particular concern in the commercial vehicle sector, where engine downsizing and electrification have limited benefit due to the vehicle duty cycle. In this paper, we present results from an experimental program to investigate the impact of dual fueling a heavy duty engine on hydrogen and diesel. Hydrogen is potentially a zero carbon fuel, if manufactured from renewable energy but could also be manufactured on the vehicle through steam reformation of part of the liquid fuel. This opens a novel pathway for the recovery of waste heat from the exhaust system through the endothermic steam reformation process, improving the overall system efficiency. For these concepts to be viable, it is essential the dual fueled combustion system is both thermally efficient, and does not increase toxic emissions such as NOx. The test program reported studied the impact of hydrogen injection into the engine intake system with and without Exhaust Gas Recirculation (EGR). The baseline engine was calibrated to achieve Euro VI NOx emissions with various aftertreatment strategies. The impact of displacing diesel with increased quantities of hydrogen was studied. The results are compared with a conceptual model of the hydrogen - diesel combustion process presented in the paper.