A mathematical model to predict the evolution of the latex particle size distribution in an emulsion polymerization reactor was developed. The mathematical framework is based on the population balance approach. It is general in framework, readily expandable to incorporate the physiochemical phenomena of interest to the reacting system of interest. The model includes such mechanistic details as (1) particle generation from radicals entering micelles; (2) particle size dependence of the radical entry mechanism; (3) coupling of the radical concentration in the aqueous phase and the particle phase; (4) determination of the particle phase radical concentration by radical entry into, exit from, and termination inside the particle; and (5) thermodynamic equilibrium between the monomer concentration in the aqueous phase and the particle phase. The model was solved efficiently with orthogonal collocation. Dynamic simulations were compared with experimental data taken from the literature for the emulsion polymerization of styrene (monomer), potassium persulfate (initiator), and sodium dodecyl sulfate (emulsifier). The variables considered were the total number of particles formed, duration of the nucleation period, conversion at the end of the nucleation period, variation of the monomer volume fraction in the particles with time, and conversion-time curves for different monomer, initiator, and emulsifier concentrations. Close agreement was found between the simulations and the experimental data. (C) 2008 Wiley Periodicals, Inc.