In this study, the ambient air NOx pollution was assessed in details in a pilot area, with a length of 26 km in east-west direction and a width of 10.5 km in north-south direction, within the area of Kocaeli province, the most industrialized region of Turkey. The area was chosen according to distribution of population, use of motor vehicles, distribution and types of industrial institutions and the meteorological conditions. Dispersion modelling approach was applied for the determination of contributions of the different source types to NOx pollution in the ambient air. In the modelling studies, an USEPA-approved Gaussian dispersion model, namely, the Industrial Source Complex Short Term (ISCST3) Model was used. The contributions of the point sources (industrial plants), area sources (residences) and line sources (motor vehicles) were estimated to be 88 %, 6 % and 6 %, respectively, by the model. On the other hand, the maximum concentration point (56.08 mu g/m(3) annual average) was predicted to be in the western area of Kocaeli, where a huge petrochemical plant and a refinery are placed.