The kinetics of surfactant-free, ab initio, batch emulsion polymerization of styrene mediated by the nitroxide SG1 ((N-tert-butyl-N-1-diethylphosphono-(2,2-dimethylpropyl) nitroxide) in the presence of a poly(sodium acrylate) macroalkoxyamine playing the role of both the initiator and the stabilizer were Studied, and the influence of the compartmentalization of the propagating radicals on the monomer conversion rate was discussed. It was pointed out that the local monomer concentration within very small particles has to be considered with great care. The results showed that free nitroxide was not compartmentalized and that the kinetics were not particularly affected by particle size, even for very small particles below 66 nm in diameter.