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A simple laboratory method has been developed for the determination of the total effective concentration of chain-stopping antioxidant species present in new and in used lubricants. The principle of the method is based upon the titration of antioxidant species by peroxy radicals formed at a constant rate from the decomposition of a free-radical initiator. Examples are presented. In agreement with theoretical predictions, it is observed that the decay of antioxidant species takes place in used samples before significant changes in the other properties of the lubricant occur. Rapid degradation of the lubricants takes place only when the antioxidant species decay to a low level. Accordingly, it is suggested that the method may be useful in establishing correlations between laboratory tests and service use, and in the development of predictive chemical models for the useful lifetimes of lubricants. (Author)