AbstractThe binary cluster integral, β, was computed from intrinsic viscosity data. Subtracting from β the polar contribution, βe, calculated from YRCR theory,9 the nonpolar interaction parameter, βn, was found. The calculations were performed for poly(vinylacetate) and poly(methyl methacrylate), each in 16 solvents. The correlation between βn and the solvent solubility parameter, δ1, was found to be similar to that reported8,17 for solutions of natural rubber, cis‐polybutadiene and for poly(vinyl chloride). This correlation can be crudely approximated by the formula where E and F are functions of the ill‐defined symmetry of the solvent molecule and δm is the δ1 value for the local maximum of the function. At δ1 = constant, the more spherical is the molecule, the higher is the βn value. It was shown that for most cases separation of the solvent into two classes (linear and nonlinear) is sufficient. This βn behavior finds support in the Funk and Prausnitz6 report on aromatic–saturated hydrocarbon mixtures and in the theoretical calculations of Huggins.21,22