This paper investigates the emissions of the unburned gaseous fuels of a heavy-duty diesel engine converted to operate under natural gas (NG)-diesel and hydrogen (H-2)-diesel dual fuel combustion mode. The detailed effects of the addition of H-2, NG, engine load, and engine speed on the exhaust emissions of the unburned H-2, methane (CH4), and carbon monoxide (CO) were experimentally investigated. The combustion efficiencies of CH4 and H-2 supplemented were also examined and compared. The emissions of the unburned gaseous fuels and their combustion efficiency were affected by the engine load, the amount of gaseous fuel added, and the engine speed. Among these, the engine load was recognized as the main factor dominating the emissions and combustion efficiency of the gaseous fuels especially when added at a small amount. The maximum emissions of H-2 or CH4 were observed when the volumetric concentration of H-2 or NG in the intake mixture reached 4% or 3%, respectively. Increasing the addition of H-2 or NG over 4% or 3% started to dramatically improve the combustion efficiency of gaseous fuels with the maximum combustion efficiency of H-2 and CH4 observed with the maximum addition of gaseous fuels. Although having significantly different combustion characteristics, the combustion efficiencies of H-2 and CH4 of H-2-diesel and NG-diesel dual fuel engines were comparable especially at high load operation. When added at a small amount, the combustion efficiency of CH4 was comparable but always slightly lower than that of H-2. This was due to the increased CO emissions of the NG-diesel dual fuel engine. The CO emissions originating from diesel fuel also contributed to this difference. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.