A Monte Carlo approach has been developed to simulate the miniemulsion polymerization of n-butyl acrylate with a water-soluble monomer, 2-hydroxyethyl methacrylate. The proposed simulation takes into account all the reactions in the aqueous and organic phases, as well as the entry of oligoradicals into the polymer particles by absorption and precipitation. The effect of the water-soluble monomer on the polymerization rate and on the molecular weight distribution of the polymer in the aqueous and organic phases has been studied. The addition of the water-soluble monomer retards the polymerization, though it had no significant effect on the molecular weight of the polymer produced in the particles; however, it increased the concentration of water-soluble polymer and its molecular weight. By this approach, it is possible to extract detailed information of polymer in the aqueous phase, such as the copolymer composition distribution.