The consumption of various p-phenylenediamines during the oxidation of natural rubber vulcanizates has been investigated in the temperature range of 80–120° C. Oxygen absorption was used to follow the oxidation. Diaryl-p-phenylenediamines were consumed mainly in a termination reaction during the constant rate region of the oxygen absorption, where five to six molecules of oxygen were absorbed per molecule of diamine consumed. The constant rate stage of the oxygen uptake was found to be about twice as long as the time of the constant rate consumption of the diamine, indicating that the reaction product also possessed antioxidant activity. The effect of the nature of the curing system, carbon black, structure of the diamine, initial concentration of the diamine, and temperature of oxidation were studied. Dialkyl- and alkyl-aryl-p-phenylenediamines were consumed predominantly by a direct, relatively fast reaction with oxygen, the rate being dependent on the structure of the diamine and also on the nature of the curing system. The oxidation products formed exhibited antioxidant activity, those of N-isopropyl-N′-phenyl-p-phenylenediamine being particularly effective.