A series of anionic organic peroxides were synthesized and used to create polystyrene nanoparticles in sodium dodecyl sulfate and dodecyltrimethylammonium bromide microemulsions. A plot of log(polymer molecular weight) vs log O/W partition coefficient is linear with a slope of -0.9 for initiators that have similar free radical generation efficiencies, suggesting that very few of the free radicals from peroxide solubilized in water react with monomer to form polymer. This indicates that the radicals formed at the interface are far more effective in initiating the polymerization than those generated outside the microemulsion droplets.