Thermal oxidation and copper-catalyzed thermal oxidation of polypropylene were studied over a range of elevated temperatures. The apparent activation energy for both of these processes, based on t10 induction periods, was 27 kcal./mole. Measurement of the time (t10) required for interaction of 1 g. of polymer with 10 cc. of oxygen was found to be a convenient and expedient technique for estimating the induction period, since this amount of oxygen is sufficient to cause deterioration of physical and dielectric properties of the polymer. In the absence of copper, stabilization of polypropylene comparable to that achieved with polyethylene was observed when antioxidant concentration was increased proportionately to compensate for the larger number of oxidation-susceptible tertiary carbon atoms in polypropylene. However, it was found that thermal antioxidants, even in high concentration, were ineffective protectants for polypropylene when copper was present. Conventional copper chelators and metal deactivators were, at best, only slightly effective in suppressing oxidation catalyzed by copper and usually were unsatisfactory for a variety of other reasons. Oxamide, however, and particularly its less volatile, high melting, disubstituted derivatives, functioned cooperatively with a wide variety of antioxidant systems and were found to comprise a highly effective and useful family of inhibitors of the copper-catalyzed oxidation of polypropylene.