Abstract The present study provides a comprehensive investigation on the determination of the primary particle size distribution in the suspension “powder” polymerization of vinyl chloride. The primary particle size distribution inside the polymerizing monomer droplets is determined by the solution of a population balance equation governing the nucleation, growth, and aggregation of the primary particles. The stability of the colloidal primary particles is expressed in terms of the electrostatic and steric stabilization forces. The primary particle stability model includes the effects of agitation, temperature, electrolyte as well as primary and secondary stabilizer concentrations. It also includes both diffusive and shear-induced particle destabilization mechanisms. The proposed stability model is shown to accurately describe existing experimental data on particle number, mean particle size and particle size distribution for both bulk and suspension vinyl chloride polymerizations. The primary particle population balance model can predict the critical monomer conversion at which massive particle aggregation occurs leading to the formation of a continuous network of primary polymer particles inside the polymerizing monomer droplets. A detailed investigation on the predicted critical monomer conversion is carried out including its dependence on the rate of agitation, temperature, electrolyte concentration, as well as the concentrations of the primary and secondary stabilizers.