A novel approach is developed to estimate the interaction energy density, B, of polymer solutions by the melting point depression method using a crystallizable molecule as the probe. It rests on the classical Flory-Huggins lattice model of polymer solutions, and considers the polymer-solvent interaction parameter to be concentration dependent. The ternary system dimethyl terephthalate (DMT)/bis(2-ethylhexyl)adipate (DOA)/poly(vinyl chloride) (PVC) is chosen for the present study. The data of B for the three possible binary interactions are reported as the functions of composition at elevated temperatures. For a plasticized PVC resin containing 20 vol % DOA, the B is estimated to be 6 MJ m(-3), which is approximately 2 MJ m(-3) higher than the B value cited in the literature for the PVC mixed with almost 100% DOA. The thermodynamic stability of the ternary system is discussed.