The vapour pressures of mixtures of the two solutes α-methyl styrene dimer and poly-α-methyl styrene in the two solvents α-methyl styrene and cumene were measured at 65°C. From the activity of the solvent, the lattice theory parameter w/kT was calculated for each solution using the quasi-chemical approximation of Guggenheim. In these calculations, two values of r were used at z= 8, viz., the degree of polymerization of the solute and the ratio of the molar volumes of solute to solvent. For a given composition in a given solvent, the value of w/kT for the dimer was significantly lower than that for the polymer with the same choice of r. It is suggested that the value of w/kT for the polymer + solvent systems includes an additional factor to the interchange energy.