A differential pressure technique was developed to specify the monomer partitioning coefficients in latex systems at monomer concentrations up to saturation. Differential pressures were measured on addition of the monomer either to water or to latex. Distribution of monomers between the polymer-particles and aqueous phase was determined using both experimental data sets. Partition coefficients of vinyl acetate and butyl acrylate in the latexes based mainly on the corresponding monomers were evaluated at 65 degreesC. The data were correlated by a new model considering the thermodynamic equilibrium between two liquid phases of the latex system. Monomer activity in the polymer-particle phase is calculated in the frame of the Flory-Huggins theory, the interfacial contribution being taken into account in terms of the Gibbs-Thomson model. The monomer activity in the aqueous phase is correlated by the NRTL equation. Model parameters were determined independently from the pressure-concentration data for the monomer-water and monomer-polymer binaries.