The present work has the aim of developing a semi-empirical correlation to estimate the NO₂/NOx ratio as a function of significant engine operating variables in a modern EURO VI diesel engine.The experimental data used in the present study were acquired at the dynamic test bench of ICEAL-PT (Internal Combustion Engine Advanced Laboratory at the Politecnico di Torino), in the frame of a research activity on the optimization of a General Motors Euro VI prototype 1.6-liter diesel engine equipped with a single-stage variable geometry turbine and a solenoid Common Rail system. The experimental tests were conducted over the whole engine map.A preliminary analysis was carried out to evaluate the uncertainty of the experimental acquired data and the NO₂/NOx ratio. The main engine variables which were expected to be related to the NO₂ formation were then identified, and a second-order polynomial model was introduced to model the NO₂/NOx ratio as a function of the abovementioned related engine variables. All the input variable combinations were considered in the investigation, in order to identify the most important for the evaluation of the NO₂/NOx ratio.Finally, a simplified exponential correlation was established to estimate the NO₂/NOx ratio, that could be used in several applications, including feed-forward model-based combustion control. The model was tested and validated on a NEDC cycle (New European Driving Cycle).