The mechanism of the inhibiting effect of diphenylamine during the oxidation of two base stocks of hydraulic oils were studied at 120, 130, and 140 degrees C. The key reactions in the mechanism of the action of the inhibitor were identified, and the values of the kinetic parameters and their activation energies were determined. The kinetic parameters were calculated, and the salient kinetic features of the inhibited oxidation of the two specimens were compared at 70 degrees C, the temperature of accelerated testing of hydraulic oils. The results of this prediction were analyzed, and the reasons for differences in the oxidation stability of the samples examined were established.