The nitrogen dioxide (NO₂) emissions of compression ignition diesel engines are usually relatively small, especially when operated at medium and high loads. Recent experimental investigations have suggested that adding hydrogen (H₂) into the intake air of a diesel engine leads to a substantial increase in NO₂ emissions. The increase in NO₂ fraction in the total NOx is more pronounced at lower engine load than at medium- and high-load operation, especially when a small amount of H₂ is added. However, the chemistry causing the increased NO₂ formation in H₂-diesel dual-fuel engines has not been fully explored.In the present work, kinetics of NO and NO₂ formation in a H₂-diesel dual-fuel engine are investigated using a CFD model integrated with a reduced hydrocarbon oxidation chemistry and an oxides of nitrogen (NOx) formation mechanism. A low-load and a medium-load operating condition are selected for numerical simulations. The experimental trends of NOx emissions are reproduced with the numerical model. The effect of in-cylinder chemical and thermal conditions on the formation of nitric oxide (NO) and NO₂ is studied through a set of numerical simulations. It is found that the evolution of in-cylinder HO₂ radicals and in-cylinder mixture temperature are both responsible for the observed trends in NO₂ emissions. The presence of increased levels of HO₂ and reduced temperature of the combustion products provide a favorable environment for conversion of NO to NO₂. Although the HO₂ radicals necessary for this conversion are produced mainly during the mixing controlled diffusion combustion of H₂ and diesel, the conversion of NO to NO₂ is mainly observed after the completion of the main combustion process.