The present study describes the development of a nonhomogeneous two-compartment model for the prediction of particle size distribution in a semibatch emulsion ter-polymerization reactor. The multicompartment model accounts for spatial variations of particle size distribution (PSD) in the reactor due to nonideal mixing conditions. A comprehensive emulsion polymerization model is applied to each compartment, which allows the calculation of the various species concentrations in the aqueous and particle phases in each compartment Moreover, a particle population balance equation is solved for each compartment to determine the individual PSDs as well as the overall PSD in the reactor. The effects of the two compartment nonhomogeneous model parameters, that is, the volume ratio of the two compartments, the compartment exchange flow rates, and the partitioning of the monomer and initiator feed streams into the two compartments, on the overall polymerization rate and PSD are analyzed in detail. It is shown that depending on the selected values of the two compartment model parameters, the :Overall PSD in the reactor can significantly vary (i.e., from a narrow and/or broad unimodal distribution to a bi- and/or multimodal PSD). Small compartment exchange flow rates, uneven monomer and initiator feed partitioning, or unequal compartment volumes can result in very different PSDs in the two compartments. Moreover, it is shown that for a range of parameter values in the two-compartment model (i.e., reflecting the degree of reactor nonhomogeneity), the calculated overall PSD in the industrial-scale reactor can be unimodal but significantly broader than the respective PSD calculated by the homogeneous one-compartment model.