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The proportion of nitrogen dioxide in the engine-out emissions of a Diesel engine is of great importance for the conversion of the total oxides of nitrogen (NOX) emissions in SCR catalysts. Particularly at lower engine loads and lower exhaust temperatures an increase of the already low NO2/NOX fraction will enhance the SCR operation significantly. For this purpose, the understanding of the NO2 formation during the Diesel combustion and expansion stroke is as substantial as being aware of the different thermodynamic impacts and engine operating parameters that affect the formation process. To determine the influences on the NO2 emission level several variation series were performed on a single-cylinder research engine. Especially the charge dilution parameters like the air-fuel ratio and the EGR rate as well as the injection parameters could be identified to be decisive for the NO2 formation.