A mathematical model for the development of the particle morphology in emulsion polymerization has been developed. The model accounts for phase separation leading to cluster nucleation, polymerization, polymer diffusion, and cluster migration. The model has been used to simulate batch emulsion polymerizations of methyl methacrylate on a polystyrene seed for which-experimentally determined particle morphologies have been reported. A good agreement between experimental results and model predictions was achieved. On the other hand, sensitivity analysis showed that the final particle morphology was not significantly affected by either the initial cluster volume or the cluster nucleation rate constant.