A comprehensive mathematical model is developed to quantify the effect of the oxygen concentration on the polymerization rate and particle size, distribution in an unseeded vinyl chloride batch emulsion polymerization reactor. Particle formation is assumed to proceed by both the homogeneous and micellar nucleation mechanisms. Dynamic species mass balances are derived to. follow the evolution of the polymerization rate and particle size distribution (PSD) in the batch reactor. It is shown that, at low initial oxygen concentrations:, the polymerization rate increases with the oxygen concentration. On the other hand, the average latex particle size respect to the initial oxygen concentration. The experimental exhibits a U-shaped. behavior with respect to the initial oxygen concentration. The experimental observations on. the polymerization rate and the average particle size are explained by the combined role of oxygen as an inhibitor and a radical generator through the formation and subsequent decomposition of vinyl polyperoxides. The predictive capabilities of the present model are demonstrated. through the successful simulation of experimental data on monomer conversion and PSD obtained from an industrial pilot-plant batch PVC reactor operated with various initial oxygen and initiator concentrations.