The oxidation of lactic acid by chromic acid was studied at high concentrations of HClO4, HNO3, and H2SO4. It was observed that the reaction rate depends not only on the acidity of the medium, but also on the nature of the mineral acid. The use of the Bunnett and Bunnett–Olsen criteria and the excess acidity concept allows us to propose a reaction mechanism of the A-2 type, the rate-determining step being a bimolecular process in which water participates as a nucleophile. The enthalpy and entropy of activation were determined; the decrease of the magnitude of these parameters when the acidity of the medium was increased suggests the existence of a compensation effect.Keywords: chromic acid, oxidation, lactic acid, acidic media.