An experimental study of the thermal transition behavior and blend miscibility of hydrogenated acrylonitrile-butadiene rubber (HNBR) is described. Glass transition temperatures were determined as a function of composition. It was found that copolymers in the 40-50% composition range crystallized as did low acrylonitrile copolymers. The tendency for the former was much greater. For the former, HNBRs are miscible with chlorinated polyethylene and chlorosulfonated polyethylene. They are immiscible with polychloroprene, ethylene acrylate copolymers and various hydrocarbon rubbers.