The addition of hydrogen (H-2) into the intake air of a diesel engine was found to significantly increase the emissions of nitrogen dioxide (NO2). Previous research demonstrated a strong correlation between the emissions of NO2 and unburned H-2 in exhaust gas. However, the mechanism whereby H-2 addition in increasing NO2 formation in a H-2-diesel dual fuel engine. Previously has not been investigated. This research numerically verified the hypothesis that the increased NO2 emissions observed with the addition of H-2 was formed through the conversion from NO to NO2 during the post combustion oxidation process of the unburned H-2 when mixed with the hot NO-containing combustion products. A variable volume single zone model with detailed chemistry was applied to simulate post-combustion oxidation process of the unburned H-2 and its effect on NO2 emissions. The mixing of the unburned H-2 with the NO-containing hot combustion products was found to convert NO to NO2. Such a conversion is promoted by the hydroperoxyl (HO2) radical formed during the oxidation process of the H-2. The factors affecting the NO2 formation and its destruction include the concentration of NO, H-2, O-2, and the temperature of the bulk mixture. When H-2 and hot NO-containing combustion products mixed during the early stage of expansion stroke, the NO2 formed during H-2 oxidation was later dissociated to NO after the complete consumption of H-2. The complete combustion of H-2 exhausted the source of HO2 necessary for the conversion from NO to NO2. The mixing of H-2 with combustion products during the last part of the expansion stroke was not able to convert NO to NO2 since the temperature was too low for H-2 to oxidize and to provide the HO2 needed. The bulk mixture temperature range suitable for meaningful conversion from NO to NO2 aided by HO2 produced during the oxidation of H-2 was examined and presented. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.