A two-compartment population balance-model has been developed for taking into account the large spatial variations of the local turbulent kinetic energy, in order to predict the evolution of droplet sizes in a high holdup (i.e., 47-50 vol%) suspension polymerization system as a function of the most important process conditions. Phenomenological expressions were applied for describing the breakage and coalescence rates as a function of the local energy dissipation rate and physical properties of the system. Computational fluid dynamics (CFD) simulations were used for estimating the volume ratio of the impeller and circulation regions, the ratio of turbulent dissipation fates and the exchange now rate of the two compartments at different agitation rates and continuous phase viscosities. A satisfactory agreement was obtained between theoretically and experimentally determined drop size distributions.