Carbon-13 kinetic isotope effect in the decarbonylation of lactic acid of natural isotopic composition in sulphuric acid diluted with water in M(H₂O)/M{H₂SO₄) molar ratio equal to 2.2 has been studied in the temperature range 80-130°C and found to be normal. The absolute values and the temperature dependence of the experimental ¹³C-K.I.E. are in agreement with the absolute values and the temperature dependence of the theoretical ¹³C-K.I.E. calculated under the assumption that one frequency corresponding to the carbon-oxygen bond broken in the decarbonylation reaction is lost in the course of activation of lactic acid molecules. The chemical side reactions leading to the abnormal temperature dependence of the carbon-13 and carbon-14 isotope fractionation, observed in the course of decarbonylation of lactic acid in concentrated sulphuric acid, have been suggested.