This study looks at the thermal behavior of mixtures of nitrile-butadiene rubber (NBR) with a resol or a novolac resin. When heated in the 200-300 degreesC range, NBR became hard and insoluble because of exothermic curing and cyclizing chain reactions involving the double bonds of its butadiene units and the nitrile groups of its acrylonitrile units. The NBR proved to be compatible with the novolac but not the resol. Heating increased the modulus of elasticity of the mixtures, which was due to crosslinking of the resin and the rubber. Preliminary linking of the resin resulted in phase separation in the novolac and prevented curing of the rubber when the mixture was heated.