The kinetics and mechanism of polymerization of miniemulsions prepared from 1% polystyrene in styrene solutions were investigated by conducting reactions with a varying potassium persulfate initiator concentration, and the results were compared with those for miniemulsions prepared in the absence of polymer. The addition of the polystyrene to the miniemulsions was shown to greatly increase the number of polymer particles produced and the overall polymerization rate obtained. The final number of polymer particles was determined for both systems and was shown to vary with the 0.31 power of the initiator concentration in the absence of polymer but was independent of the initiator concentration for miniemulsions prepared from 1% polystyrene in styrene solutions. The final particle size distributions also differed when polystyrene was added to the miniemulsions. The miniemulsions prepared in the absence of polymer produced negatively skewed particle size distributions, the particle size decreasing with increasing initiator concentration. The miniemulsions prepared from 1% polystyrene in styrene solutions produced bimodal particle size distributions for all but the highest initiator concentration employed. By treating the system in an analogous fashion to seeded emulsion polymerization, it was demonstrated that the mechanism for polymerization of miniemulsions prepared from 1% polystyrene in styrene solutions proceeds by radical entry into highly monomer-swollen, preformed polymer particles. Calculation of the average number of radicals per particle for this system revealed that Smith-Ewart case 1 and/or case 2 kinetics were obeyed.