The oxides of nitrogen (NOx) 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.