A mathematical model for microemulsion copolymerization systems has been developed and compared with experimental conversion vs. time data for the vinyl acetate/butyl acrylate system. The main features of the model include micellar and homogeneous nucleation and thermodynamic equilibrium for the calculation of monomer partitioning between the phases. Simulations were made using this model with reported values for the kinetic parameters except for propagation rate constants in the water phase and radical capture by particles and micelles coefficients which were estimated. Simulations using the model showed good agreement with experimental data.