Microemulsion polymerizations are attractive for investigating compartmentalization effects in heterogeneous reversible addition - fragmentation chain transfer (RAFT) polymerizations because the propagating radicals are segregated into surfactant stabilized polymer particles, which drastically reduces the effects of biradical termination, Also, microemulsion polymerizations do not involve the large monomer droplets that are present in emulsion and miniemulsion polymerizations. RAFT microemulsion polymerizations of butyl aery late with a high water solubility chain transfer agent, methyl-2-(O-ethylxanthyl)propionate (MOEP), and a low water solubility chain transfer agent, methyl-2-(O-dodecylxanthyl)propionate (MODP), were investigated to determine the effect of chain transfer agent compartmentalization on the control of the polymerization. The partitioning of the chain transfer agent into the polymer particles is shown to be the primary factor in determining the final properties of the polymer and latex. Polymerizations with the high water solubility chain transfer agent MOEP produced stable latex nanoparticles (20-30 nm in diameter) containing low polydispersity poly(butyl acrylate) of predetermined molecular weight, whereas polymerizations with the low water solubility chain transfer agent MODP produced stable latex nanoparticles containing polymers with a multimodal molecular weight distribution.