Hydrogen (H-2) emissions characteristics of H-2-diesel dual fuel engine were measured using a 2004 turbocharged heavy-duty diesel engine with H-2 supplemented into the intake air. The emissions of H-2 were measured using an Electron Pulse Ionization (EPI) Mass Spectrometer (MS). The effect of the amount of H-2 added, the engine load, and diesel fuel flow rates on the emissions of H-2 and its combustion efficiency in the engine were investigated. The addition of H-2 under high load operation was notable for its ability to obtain high H-2 combustion efficiency and improve brake thermal efficiency. However, the addition of H-2 at low load resulted in high emissions of H-2 due to the failure to initiate and support a sufficiently vigorous flame for the complete combustion of H-2 present outside the diesel spray plume. The maximum H-2 emissions of 1.4% (volume in dry exhaust gas) were observed with the addition of 6% H-2 at 10% load. In comparison, the maximum H-2 emissions of 0.13% were observed when operated at 70% load with the addition of 6% H-2. The slip of a large percentage of H-2 at low load operation was shown to deteriorate the potential of H-2 in improving the brake thermal efficiency. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.