The objective of this study is to develop a physical model that accurately accounts for the nucleation, coagulation, and condensation processes in the formation of particulate matter (PM) inside the exhaust plume of the diesel-fueled vehicles. The PM concentration has been predicted based on the fuel sulfur content, fuel-to-air ratio, exhaust flow rate, and the ambient conditions. It was predicted that the critical nucleus diameter of the particles decreased by approximately 30% and the number concentration increased by a factor of 6 with the increase in relative humidity from 10% to 90% for a fuel with 50 ppm sulfur content. The parametric studies suggested that the condensation effects are very important near the stack. Ignoring the contribution from condensation term decreased PM count median diameter from 52 to 10 mn. A fair agreement is observed between the numerically predicted PM size distribution and concentration and the experimentally measured values. (C) 2002 Elsevier Science Ltd. All rights reserved.