A method of estimating the thermal index of electric insulators is suggested by its correlation with quantities which can be determined from TG curves : the temperatures required to obtain given fractions of their weight loss; the temperature corresponding to the maximum rate of decomposition; the total energy of activation of thermal decomposition; or the partial energy of activation of process around a given fraction of weight loss. Using five resins with different structures, the optimizing programme applied gave the best correlation of the thermal index with the temperature corresponding to weight loss fractions of 0.14 or 0.15, if one-parameter equations were used.