Xanthates ([1-(O-ethylxanthyl)ethyl]benzene (CTA1) and [1-(O-trifluoroethylxanthyl)ethyl]benzene (CTA2)) have the capacity to control the molecular weight distribution in emulsion polymerizations to produce very small nanoparticles below 20 nm. We form stable translucent polystyrene latexes using surfactant (sodium dodecyl sulfate, SDS) and a small amount of pentanol as cosurfactant. The high CTA concentration results in a greater retardation in rate until consumption of all the RAFT agent. With an increase in CTA1 the particle size decreases from 38 to 8 nm and the particle number concentration Nᴄ increases from 2 × 10¹⁸ to 2 × 10²⁰ particles/L. Although an increase in Nᴄ should in principle lead to a faster rate of polymerization, we observe a greater retardation in rate with increasing CTA. The higher Ctr,RAFT of CTA2 results in a greater initial retardation until consumption of all the RAFT agent and particle diameters lower than 5 nm and at high concentrations of CTA2 diameters that are not measurable. Kinetic simulations solving the Smith−Ewart equations explain the anomaly between R• (formed from the fragmentation of the R group from the RAFT agent) acting to nucleate micelles and terminate radicals within particles. The small and mobile R• radicals can exit particles, re-enter micelles or other particles, re-exit until they either nucleate micelles, or terminate with propagating polymeric chains. This process of exit and re-entry is similar to limit 3 in a conventional emulsion polymerization. The higher micelle nucleation rate through initiation within micelles by R• radicals results in smaller and a greater number of particles. Exit is the dominant mechanism for greater nucleation and retardation.