[en] Intramolecular ¹³C kinetic isotope effects in the decomposition in dioxane solution of oxalic acid to carbon dioxide and formic acid were determined at several temperatures between 102 and 141degreeC. The isotope effect is small (approximately 0.5%) and any temperature dependence is masked by the experimental imprecision (which averages about 0.09%). The method of three-element reaction coordinates (with ν₁/sup double-dagger/=0) was employed to expose possible properties of the transition state. Relatively good definition of transition state properties is obtained with several models. But, when one requires simultaneous fit of the model calculations to the results recorded here and to the intermolecular isotope effects reported earlier (''double correspondence''), the parameters are severly confined. The best double correspondence is obtained when: (a) transition state force constants are set to correspond to bond orders (near the C atom which will appear in carbon dioxide product) of C--C near 0.1, of C--O near 1.9, with that of O--H set at 0.5; (b) the relative internal coordinate displacements comprising the reaction coordinate eigenvector are C--C extension, 0.50 to 0.75; C--O contraction, 0.15 to 0.18; and O--H extension, 0.35 to 0.07. Anharmonicity and other small corrections are neglected without substantial effect on the conclusions described