When methyl methacrylate (MMA) is polymerized in aqueous suspension, it has been shown that, in fact, the reaction takes place in both dispersed and continuous phases in spite of the low solubility of the monomer in water. We study here the case where MMA and acrylic acid (AA) are used to form copolymers in an aqueous suspension. The determination of the reaction rate requires the understanding of mass transfer. On using the UNIFAC group method for the calculation of the activity coefficients, the monophasic and diphasic zones in the ternary diagram MMA/AA/water have been predicted accurately. Therefore, we have chosen, in the diphasic zone, the composition and the characteristics of the mixtures with a view to understand the mass-transfer process between the phases occurring in the reactor during the polymerization. The resolution of mass and thermodynamic balances established on each component in each phase allows us to simulate the fractional conversion of each monomer in both phases. A good agreement is observed between the experimental data and the simulation results.