This paper investigates the impact of hydrogen (H-2) addition on the exhaust emissions of a 2004 Mack MP7 355E heavy-duty diesel engine. As expected, the addition of H-2 reduced substantially the emissions of particulate matter (PM), unburned hydrocarbon (HC), and carbon dioxide (CO2). However, the effects of H-2 addition on the emissions of carbon monoxide (CO) and oxides of nitrogen (NOx) depended on the amount of H-2 added and engine load. For measurements using the 13-mode European Stationary Cycle (ESC), the addition of 2% and 4% H-2 (vol.% in intake mixture) increased substantially the nitrogen dioxide (NO2) emissions but reduced the emissions of nitric oxide (NO). The addition of H-2 only minimally affected NOx emissions. The detailed effects of H-2 addition on the exhaust emissions were investigated for various loads at 1200 rpm. The addition of H-2 at low load mildly influenced NOx emissions, with the exception of 10% load. Adding over 4% H-2 at 10% load began to reduce NOx emissions. When operated at medium to high loads, the addition of a relatively small amount of H-2 slightly reduced NOx emissions. The expected increase in NOx emissions was observed only with the addition of a large amount of H-2 at medium to high loads. When operated at full load, the addition of H-2 had a negligible effect on NOx emissions. In a few cases, a minor change in H-2 flow rate suddenly increased NOx emissions. By further increasing or reducing the amount of H-2 added, NOx emissions resumed to the expected values. This discrepancy was attributed to the unexpected change in the flow rate of the recirculated exhaust gas. Based on the data obtained, it seems infeasible to substantially reduce the exhaust emissions of this diesel engine through the addition of a small amount of H-2 such as that produced on-board using a small H2O electrolyzer, or diesel fuel reformer. (C) 2011 Published by Elsevier Ltd.