The correlation between the thermodynamic compatibility of cis-polyisoprene rubber and correlated butyl rubber, and the physicochemical properties of vulcanizates of their blends are shown. The maximum strength of the vulcanizates is attained in micro-heterogeneous systems under conditions close to the critical conditions for blend formation. The important part played by the vulcanizing group, which msut ensure covulcanization of the rubbers at the phase interface, is indicated.