AbstractVinyl chloride monomer was polymerized in an emulsion system in the presence of a redox catalyst and relatively low concentrations of emulsifier. The effects of sodium bisulfite, potassium persulfate, and commercial sodium lauryl sulfate concentrations on particle initiation were determined by response‐surface experimentation. A regression equation was developed which describes the number of particles per unit volume initiated in this vinyl chloride emulsion system. At subcritical emulsifier concentrations, the number of particles initiated was dependent primarily upon the emulsifier and sodium bisulfite concentrations and to a lesser degree upon the persulfate concentration. As the critical micelle concentration was approached, an apparent change in the particle initiation mechanism was observed, in that the sodium bisulfite concentration ceased to have an effect. In this system the change occurred at a concentration of 0.005% emulsifie