The group contribution method of UNIFAC is used to investigate the influence on adhesion of thermodynamic compatibility between the filler surface and the polymeric matrix in filled polymeric composites. Compatibility is enhanced between polymers and mineral surfaces through the use of silane coupling agents of varying chemistry, in this study, glass beads were treated with ten different organofunctional silanes intended to induce differences in interfacial strength. Interfacial strength measurements were obtained from tests in which single, silane-treated glass beads were embedded in rectangular poly(vinyl butyral) specimens subjected to uni-axial stress until interfacial failure occurred at one of the poles: of the sphere. The UNIFAC method was used to estimate the Gibbs free energies of mixing using the chemical structure of the polymer repeat unit and each of the silane organofunctional groups, and these values were correlated with the measured interfacial strengths. The results indicate that enhanced interfacial strength corresponds to systems with more favorable thermodynamic mixing.