A mathematical model is developed to describe the reaction behavior of emulsion copolymerization systems where significant polymerization occurs in both the latex particle and aqueous phases. 2-Hydroxyethyl methacrylate (HEMA) and styrene system was used to illustrate the development of a batch reactor. The model considers the gel and glass effects and monomer transfer. The principal model parameters are taken from the literature. Copolymerization reactivity ratios were estimated using a nonlinear least-squares procedure. Model predictions have been compared with experimental data on monomer conversion.