The kinetics of the dispersion copolymerization of styrene and butyl acrylate in an ethanol-water medium was investigated in a batch stirred glass reactor. The effect of the stabilizer concentration, ethanol/water ratio, styrene/butyl acrylate ratio, temperature, and initiator concentration on the particle size distributions, conversion-time curves, and molecular weight distributions was investigated. It was found that the polymerization started in the continuous medium and, depending on the process conditions, the main polymerization locus shifted to the polymer particles. The more pronounced the shift, the higher (M) over bar(w) and PI. The shift was more marked when the particles were small and when the solubility of the copolymer chains in the continuous medium was low. This solubility was the key factor in the process, as it also substantially affected the particle size.