The degradation kinetics of thermoplastic Poly Ether Imide (PEI) has been studied by thermogravimetric analysis, a kinetic model based on phenomenological considerations was developed for the pyrolitic degradation of PEI in nitrogen. It was observed that, when isothermal tests were performed, the amount of polymer degraded is function of the degradation temperature, while in constant heating experiments, the fate of degradation was a function of the heating rate. The results of such analysis were applied to obtain long term data from short term experiments as an engineering approach to evaluate the thermal resistance of this polymer.