The kinetic features of auto- and initiated oxidation of four basestock specimens of hydraulic oils were studied at 140degreesC. The key reactions in the mechanism of the process were revealed, and values for the corresponding kinetic parameters were determined. A kinetic model of oxidation describing the experimental oxygen uptake and hydroperoxide buildup curves was obtained for each specimen. The kinetic models for the samples examined were analyzed and compared, and the reasons for differences in their oxidizability were rationalized in terms of the degenerate branching-chain reaction.