A mathematical model for the emulsion copolymerization of methyl methacrylate and butyl acrylate has been developed. This model, which applies the method of moments of a distribution to model the evolution of the particle size distribution, predicts the effects of the concentration and composition of anionic/nonionic surfactant systems on the polymerization process and on the characteristics of the product obtained, including particle nucleation, growth, and coagulation. Nucleation is a dynamic process in which the surfactant system affects the competition between homogeneous and heterogeneous nucleation, with simultaneous coagulative processes of precursor particles. The effect of the surfactant system on nucleation is described mathematically using a variable radical critical chain length, j(cr). The solution properties of surfactant mixtures, mainly critical micelle concentration and micelle composition, were predicted using the thermodynamics of nonideal mixtures. A good agreement between model predictions in batch and semicontinuous reactors and experimental results was found.