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Relaxation modulus measurements on uncured silica-filled polydimethylsiloxane composites were correlated in terms of the polymer /filler interfacial energy. The heat of adsorption of a model compound (octamethylcyclotetrasiloxane) on the filler surface was taken as a measure of the interaction energy. The results on uncured composites containing these silicas revealed a strong correlation between the relaxation modulus and the net heat of adsorption. Roughly, 1.5 kcal/mol difference in the net heat of adsorption per siloxane bond caused a change in the relaxation modulus of two orders of magnitude. Dispersion of the fillers was examined with the electron microscope and revealed no significant difference in the dispersion of different silicas. The effect of interfacial energetics on the relaxation modulus can be entirely lost when cross-links are introduced into the system.