This paper investigated the nitrogen dioxide (NO2) emissions of a heavy-duty diesel engine operated in hydrogen (H-2)-diesel dual fuel combustion mode with H-2 supplemented into the intake air. Preliminary measurements using the 13-mode European Stationary Cycle (ESC) demonstrated the significant effect of H-2 addition on the emissions of NO2. The detailed effects of H-2 addition and engine load on NO2 emissions were examined at 1200 RPM. The addition of a small amount of H-2 increased substantially the emissions of NO2 and the NO2/NOx ratio, especially at low load. Increasing the engine load was found to inhibit the enhancing effect of H-2 on the conversion of NO to NO2 with the maximum NO2/NOx ratio observed at lower H-2 concentration. The maximum NO2 emissions of the H-2-diesel dual fuel operation were three (at 70% load) to five (at 10% load) times that of diesel operation. Further increasing the addition of H-2 beyond the point with maximum NO2 emissions still produced more NO2 than for diesel-only operation. Based on the experimental data obtained, the engine load and maximum averaged bulk mixture temperature were not the main factors dominating the formation of NO2 in the H-2-diesel dual fuel engine. A preliminary analysis demonstrated the significant effect of the unburned H-2 on NO2 emissions. When mixed with the hot combustion product, the unburned H-2 that survived the main combustion process might further oxidize to raise the HO2 levels and enhance the conversion of NO to NO2. In comparison, the changes in the combustion process including the start of combustion, combustion duration and maximum heat release rate may not contribute substantially to the increased NO2 emissions observed. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.