Carbon-13 intramolecular kinetic isotope effects (K.I.E.) in the decarbonylation of oxalic acid in concentrated sulphuric acid, in sulphuric acid diluted with water and in fuming sulphuric acid have been reinvestigated or investigated. Intramolecular and intermolecular ¹³C K.I.E. in the decarbonylation of oxalic acid of natural isotopic composition in phosphoric acids media have been initiated also. The previous determinations of ¹³C K.I.E. in the decarbonylation of oxalic acid in concentrated sulphuric acid at 100°C have been reproduced and it has been shown that the dilution of sulphuric acid with water as wail as increase of the concentration of SO3 over the stoichiometric one (influencing remarkably the rates of decarbonylation) do not change significantly the ¹³C fractionation in this reaction contrary to the large medium and temperature effects (especially at 100°C) observed in the decarbonylations of formic and lactic acids in sulphuric and phosphoric acid media. The above results have been rationalized by strengthening the view that in the decarbonylation of oxalic acid all bonds are substantially altered in T.S. but the loss of the two single C-C and C-O isotopic bonds in T.S. is compensated by the increase of bonding between carbon and oxygen in the carbonyl group which undergoes the transformation into free carbon monoxide molecule and diminishes the ¹³C fractionation. Additional isotope effect data have been provided which show that the structure of the T.S. in the decarbonylation of bifunctional oxalic acid in sulphuric acid and in phosphoric acids media differs largely from the structure of the T.S. in the decarbonylation of formic and lactic acids in sulphuric and phosphoric acids.