A mathematical model is developed to help quantify and improve Our Understanding of the formation of particles and the evolution of the particle size distribution (PSD) in the emulsion polymerization of vinyl chloride monomer (VCM). A special feature of the model is the computation of the coupled radical number and particle size distributions by means of the zero-one-two Population balance equations. This approach is shown to be necessary when modeling processes that involve particle nucleation. Preliminary results are presented to illustrate the ability of the model in interpreting experimental data.