A method for rapidly predicting the thermal deterioration and service life of plastics, especially those used in automobiles, is based on thermal anal.; the method is supplemented with a technique to measure the thermal deterioration rate based on thermal decomposition kinetics. A method of deducing the activation energy of thermal deterioration that would take place under a realistic service environment, from information obtained at much higher temperatures is proposed. This method allows faster assessment of the thermooxidative lifetime of plastic materials. The method is illustrated by evaluation of deterioration of a glass fiber - Nylon 66 composite.