Particle formation in vinyl chloride emulsion polymerization has been investigated by means of ab initio and seeded experiments. A series of ab initio polymerizations was run in order to obtain reliable data regarding the dependence of the particle number on the emulsifier (SDS and SDBS) concentration, as well as to analyze the effect of the initiator concentration, stirring rate, and monomer-to-water ratio upon the particle number and the polymerization kinetics. Seeded polymerizations carried out at different concentrations of seed latex and surfactant made it possible to quantitatlvely evaluate the influence of these factors on the onset and extent of secondary particle formation.