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This paper describes a methodology using FTIR analysis for monitoring the degradation by oxidation of lubricants. This methodology is based on the interpretation of the viscosity increase and of the carbonyl compounds formation measured by Peak Area Increase (PAI). This physicochemical approach was applied to the development of an innovative in-house bulk oxidation test, the Modified ICOT, and to in-service oil evaluation. This work provided the opportunity to confirm hypotheses and gave new information on the oxidation process of lubricants, especially concerning catalytic action of soluble iron and relationship between viscosity increase and carbonyl compounds formation. Laboratory test results showed a good correlation between PAI and TAN on bulk-oxidized oils, in which acidity is only related to the carbonyl compounds formed during the oxidation.