Compartmentalization in atom transfer radical polymerization (ATRP) in an aqueous dispersed phase system has been investigated theoretically to understand the effects of particle size on the rate of polymerization and the degree of control oil the livingness and polydispersity index (PDI) for the system n-butyl methacrylate/CuBr/EHA(6) TREN. The simulations indicate that there exists a defined range of particle sizes where the rate of polymerization is higher than that of a bulk system and where PDI and frequency of termination remain below that of bulk polymerization. For this highly active catalyst system, the livingness of the chains is a function only of the particle size and is independent of the rate of reaction. Furthermore, simulations conducted with very low catalyst concentrations suggest that the rate of polymerization is dependent on the absolute amount of catalyst in the system rather than the steady-state Cu(I)/Cu(II) ratio that applies for bulk polymerization. At low catalyst concentrations, the rate of polymerization decreases, and the PDI increases with diminishing catalyst concentration, whereas the chain livingness is improved.