DSC studies on Bayer's Perbunan NT 1845 (NBR 18) and 3945 (NBR 39) differing in acrylonitrile mer content, showed thermooxidative transformations to start invariably at 160degreesC and to yield hydroperoxides which decomposed to initiate degradation and thermal crosslinking (FTIR evidence) via polymerization of butadiene mers. With more acrylonitrile, crosslinking was less extensive, the rate of destruction and the energy of activation were higher (NBR 18 and 39: 149.5 and 162 kJ/mol, resp.), the formation of volatile decomposition products was slower, as confirmed by Oxygen index and thermovision data, and the flammability lower, but smoke formation higher and toxicity greater. For the 18, the max. heat release rate (3570), total heat released (98), av. mass loss (42) and CO and CO2 emissions (0.097. 3.262) were much greater than those for the 39 (3115 kW/m(2), 67 MJ/m(2), 32 g/s(.)m(2), 0.082 and 2.896, resp.).