The objective of the present paper is to demonstrate that the explicit radical-particle size distribution approach correctly predicts the effect of compartmentalization on the overall reaction rates and therefore chain length averages. Modeling results for the seeded emulsion polymerization of styrene were compared with experimental results. Several experiments were carried out with systematically varied compartmentalization of radicals by varying seed latex particle numbers and the amount of initiator. The overall polymerization rate was measured using reaction calorimetry and the final particle size distribution was measured using Transmission Electron Microscopy. The results demonstrated that the model is able to predict successfully the rate of polymerization and particle size distributions as a function of time for all recipes. This proves that the model deals correctly with the effect of compartmentalization on overall reaction rates and thus on chain length averages. The work described in this paper demonstrates that the explicit radical particle size distribution approach is a powerful method for predicting emulsion polymerization kinetics and product properties, such as particle size distributions and chain length distributions.