Recently, experimental results indicating differences between the polymerization kinetics of styrene miniemulsions prepared with and without small amounts of polystyrene added to the oil phase have been reported. The work is expanded here to better define the range of conditions under which this behavior is observed. Styrene miniemulsions and conventional emulsions prepared with and without the addition of a small amount of polystyrene to the oil phase were polymerized at 70 degrees C in an automated reaction calorimeter. The results show that miniemulsions prepared with as little as 0.05 wt % polymer based on the oil phase exhibit a substantial increase in the polymerization rate and number of polymer particles produced over miniemulsions prepared in the absence of polymer. An increase in the polymerization rate was observed when either cetyl alcohol or hexadecane was used as the cosurfactant; however, the increase was greater for cetyl alcohol. In addition, the homogenization of a polymer solution without a cosurfactant was demonstrated to yield an unstable miniemulsion (i.e., a cosurfactant is required for the formation of a stable miniemulsion). Samples withdrawn before and after the polymerization of a 1% polystyrene in styrene miniemulsion (using cetyl alcohol as the cosurfactant) revealed that the initial number of miniemulsion droplets was nearly the same as the final number of polymer particles. The latter is taken as partial proof that a large fraction (approaching 1) of the initial number of droplets captured aqueous phase free radicals and became polymer particles during this polymerization.