Irganox(R)HP-136(lactone), which is a mixture of 90% of 5,7-di-tert-butyl-3-(3,4-dimethylphenyl)3H-benzofuran-2-one and 10% of 5,7-di-tert-butyl-3-(2,3-dimethylphenyl)3H-benzofuran-2-one behaves as a medium strength chain-breaking antioxidant during polypropylene oxidation at 180 and 200 o C. The critical concentration (vide infra) required to inhibit oxidation is higher than that of the phenolic antioxidant 2246. The lactone is slowly consumed during the induction period and then much faster when the critical concentration is reached. Phosphites, sulphides and phenols increase the efficiency of the lactone during polymer oxidation. The use of lactone allows the amount of phenolic and phosphorous containing stabilizers to decrease without decreasing the thermooxidative stability of the polymer.