The mechanism of the inhibiting effect of diphenylamine during the oxidation of four basestock specimens of hydraulic oils at 120 degrees C was studied. The main reactions involving the molecular inhibitor InH and its radical In-center dot were identified, and the corresponding parameters were determined. The specific features of the mechanism of the action of diphenylamine in the presence of dicumyl peroxide as an initiator were revealed. A mathematical model for the process describing quantitatively the kinetic curves of oxygen uptake under widely changing initial conditions was derived. Based on the model, the efficiencies of the diphenylamine effect on the specimens were compared.