During acrylic acid (AA)/vinyl acetate (VAc) suspension copolymerizations, AA is partitioned between the organic phase and the aqueous phase. For this reason, the AA partition coefficient in suspensions of AA/VAc/water is studied here. Partition experiments were performed at different temperatures and AA concentrations. It was observed that the AA partition coefficient ranged from 0.001 to 1.58, depending strongly on the temperature and on the aqueous phase composition. Therefore, the distribution of AA between the continuous phase and the monomer droplets may exert a significant effect upon the observed kinetic behavior during suspension polymerizations. The UNIFAC-LLE model was used to describe the observed liquid-liquid equilibrium of the ternary system AA/VAc/water and model predictions were compared to the obtained experimental data. It was observed that the UNIFAC-LLE model is able to represent AA partition coefficients qualitatively only when AA dissociation effects are taken into consideration. Additional experiments also indicated that AA partition coefficients may be very sensitive to the presence of polymer in the organic phase and to the presence of salt in the aqueous phase, as usual during actual suspension copolymerizations. (C) 2004 Wiley Periodicals, Inc.