Elastomeric tetramethylene-ethylethylene-acrylonitrile (TEAN) copolymers were prepared by quantitative selective hydrogenation of emulsion NBR polymers in solution in presence of a triphenylphosphine-liganded rhodium salt catalyst. Completeness of hydrogenation at low pressure required appreciable excess of triphenylphosphine. Physical and chemical properties of TEAN gums of varying acrylonitrile segmer content, and their sulfur- and peroxide-cured vulcanizates are characterized. As expected, both gums and vulcanizates were very resistant to attack by oxygen. The exceptionally high tensile strengths of such vulcanizates were attributed to the crystallizability of the gums on stretching. Certain peroxide-cured TEAN vulcanizates prepared from green stocks containing magnesium oxide had a combination of resistance to heat aging in air, hydrogen sulfide, and oil, suggesting their applicability to well-drilling components.