The oxides of nitrogen (NO_x) emissions of diesel engines consist of nitric oxide (NO) and nitrogen dioxide (NO₂). Although emitted at small amounts, NO₂ has higher toxicity and causes more health and environmental issues than NO. This research investigates the impact of the addition of hydrogen (H₂), natural gas (NG), and engine load on NO₂ emissions from a heavy-duty diesel engine converted to operate using dual fuel combustion mode. The substitution of a small amount of H₂ or NG for the diesel fuel substantially increased NO₂ emissions, but had a very mild impact on the combustion process. In comparison, the substitution of a large amount of H₂ and NG for the diesel fuel dramatically altered the combustion process and produced more NO₂ than the diesel-only operation, but produced less NO₂ than the addition of a small amount of H₂ and NG. A preliminary analysis revealed a firm correlation between NO₂ emissions and the emissions of the unburned H₂ or CH₄, and their relative emissions. The importance of the unburned fumigation fuels in enhancing NO₂ formation in dual fuel engines was also supported by the data reported in the literature. The portion of supplemental fuels entrained into the diesel spray plume and simultaneously burned with the diesel fuel may not contribute to the increased NO₂ emissions from dual fuel engines.