A fully hydrogenated butadiene acrylonitrile copolymer (less than 1% residual doublebonds), which provides vulcanizates with superior resistance to heat, ozone and excellent resistance to non - polar hydrocarbon fluids including those containing aggressive additives. With this level of unsaturation, Therban^® 4309-based ompounds can only be vulcanized with peroxides or high-energy radiation. The incorporation of a crosslinking co-agent is recommended for ptimized physical properties of both unaged and aged vulcanizates.High molecular weight provides for capacity of high plasticizer and filler loads.