The methods of reverse gas chromatography and successive integral sorption have been used to determine the free energies of mixing of cis-polyisoprene (SKI-3) and butadiene-nitrile copolymers (SKN-18 and SKN-26) with an α, ω-dimethacrylate (of triethyeneglycol): the values from these determinations agree satisfactorily. It has been shown that the system SKN-26-TGM-3 is compatible and thermodynamically stable over the entire range of compositionj and is characterized by negative values of ζ′ 23. The compatibility of the components is impaired as the polarity of the rubber is decreased. SKI-3 rubber is practically incompatible with TGM-3.