The effect of stirring speed (SS) on the kinetics of styrene emulsion polymerization at 70degreesC, using SDS as surfactant and KPS as initiator, was studied. The conversion (x) and weight average molecular weight (Mw) results are higher in the optimum SS range, which seems to be strongly dependent on the hydrodynamic characteristics of the reactor and the polymerization conditions. Above the optimum SS range, the number of particles and Mw are lower, therefore limited coagulation lead to a decrease in Mw caused by the increment of the free-radical entry frequency. These radicals not only come from the aqueous phase (oligomeric radicals), but also come from the radicals contained in the particles subject of coagulation (coagulative entry).