Monomer can be added during the polymerization of a microemulsion in order to increase the amount of polymer produced. Here reported are the polymerization kinetics of a microemulsion of water, n-hexyl methacrylate (C6MA), dodecyltrimethylammonium bromide (DTAB), and didodecyldimethylammonium bromide (DDAB) to which the monomer is added during polymerization. The kinetic model developed by Morgan and Kaler [Macromolecules 1997, 30, 1897-1905] is used to examine polymerization mechanisms and is extended to predict the evolution of particle size. The results show that radicals initiated prior to monomer addition do not polymerize the added monomer and that monomer partitioning, radical exit, and the number of active radicals initiated prior to an addition can limit final particle size. The small increase in average particle size after each monomer addition indicates that new particles are nucleated and grow in each step.