Aniline catalysis of the decarboxylation of oxaloacetic acid, HO2CCOCH2CO2H + CH3COCO2H + CO2, has been studied in water and ethanol as solvent at 25°. In aqueous solution a maximum occurs in the pH-rate profile at pH 3.9, indicating the participation of a single proton in the transition state. Experiments with the half ester HO2CCOCH2CO2Et, which does not decarboxylate in the presence of amines, suggest that in aqueous solution the intermediate in the reaction is a carbinolamine formed between the amine and the keto group of the acid, and not a Schiff's base as has been previously suggested. In ethanol solution the intermediate is a Schiff's base, and formation of the Schiff's base is the rate-determining step, the actual decarboxylation being a fast process.