A mathematical model has been developed for the differential microemulsion polymerization of styrene. ha the model, both homogeneous and heterogeneous nucleation mechanisms are considered. It is confirmed that the predictions of the particle size and the fractional conversion by the model are in good agreement with the experimental data. It is indicated that the particle size of polystyrene decreases with an increase in the amount of the initiator or surfactant. (c) 2007 Wiley Periodicals, Inc.