Compartmentalization of an atom transfer radical polymerization (ATRP) miniemulsion has been experimentally observed to reduce the overall polymerization rate, primarily by increasing the rate of deactivation due to the confined space effect, and compartmentalization was also found to improve the control over the polymerization and to reduce the final polydispersity index of the polymer. Compartmentalization of an ATRP system requires that the probability of having two active chains in one particle becomes sufficiently low that the particle size influences the number of active radicals (through segregation and confined space effects). This probability is determined by both the particle volume and the number of polymer chains within each particle. For a given number of chains, compartmentalization effects become evident only when the number of reactants (active polymeric radicals and the deactivating species CuBr2-tris[2-di(2-ethylhexyl acrylate)aminoethyl]amine (EHA(6)TREN)) within each particle becomes limited by decreasing particle volume. Alternatively, for a given particle volume compartmentalization effects become evident when reactants (active polymeric radicals and the deactivating species CuBr2-EHA(6)TREN) become limited by decreasing chain number. The difference between a conventional free radical polymerization and ATRP is highlighted by the opposing impact that compartmentalization has on the kinetics of the polymerizations. In a conventional system, segregation effects cause an increase in the polymerization rate, while the confined space effect dominates the kinetics in ATRP and results in a decrease in rate.