The 13C resonances of the hydrate, keto and enol forms of oxaloacetic acid in aqueous solutions at low pH, at pH 4.3 and at pH 7.3 were assigned on the basis of the J(CH) values and the chemical shifts of reference compounds. At low pH the hydrate form predominated, while only small amounts of the keto and enol forms were detected. At pH 4.3 and 7.3 the keto form was the major form. In anhydrous dimethyl sulphoxide solution only the keto and enol forms were detected, whereas on addition of water the hydrate form became predominant. On prolonged recording of the spectra at the probe temperature (30°C) citroylformic acid was formed either at low pH or at pH 7–12, whereas only traces of pyruvic acid were detected. The aldol condensation product of oxalacetic acid was detected when the spectra were recorded at 7°C before its decarboxylation to citroylformic acid. In the spectra recorded at pH 3–7 pyruvic acid and its aldol condensation product, 4-hydroxy-4-methyl-2-ketoglutaric acid, were detected together with citroylformic acid. A complete assignment of the 13C resonances of these derivatives was made.