A mathematical model for the development of particle morphology in emulsion polymerization has been developed. The polymer particles are considered to be a biphasic system comprising clusters of polymer 1 dispersed in a matrix of polymer 2. The model accounts for both polymerization and cluster migration. Polymerization of monomer 1 occurs both in the polymer matrix and in the clusters. The polymer 1 formed in the matrix diffuses instantaneously into the clusters. The clusters migrate toward the equilibrium morphology to minimize the free energy of the system. The driving forces for the motion of the clusters are the van der Waals interaction forces between the clusters and the aqueous phase and those between the clusters themselves, The effect of polymer matrix viscosity on the cluster motion is included. illustrative simulations and comparisons with experimental data are presented.