In the suspension polymerization of vinyl chloride, colloidally stable primary particles are formed inside the polymerizing monomer droplets. The nucleation, growth, and aggregation of these particles are responsible for the formation of the internal particle morphology and associated properties, such as pore-size distribution, specific surface area, and plasticizer uptake. In the present investigation, a population balance model is derived describing the time evolution of the primary particle-size distribution in the polymerizing monomer droplets. The resulting integrodifferential equation accounts for the coalescence rate of electrically charged colloidal primary particles through the use of the Fuchs modification of the extended Smoluckowski coagulation equation. Simulation studies were carried out to determine both the effects of the model parameters and to afford a direct comparison of model predictions with experimental measurements of the time evolution of the primary particle-size distribution. The dependence of the total number and of the mean diameter of the primary particles on several process variables (i.e., the ionic strength of the medium, total particle charge, temperature, initial initiator concentration, and agitation rate) is quantitatively analyzed. Finally, the predictive capability of the present model is demonstrated by a direct comparison of model predictions with the experimental data of Willmouth et al, and Tornell and Uustalu.