Aqueous or aqueous ethanolic samples of aliphatic and aromatic acids were coupled with 2-nitrophenylhydrazine hydrochloride to give acid hydrazides, which showed intense violet colors in an alkaline medium. Dicyclohexylcarbodiimide was used as a coupling agent, and a small amount of pyridine markedly enhanced the reaction. The brown blank solution was decolorized to a faint yellow by heating the alkaline solution at 60°. The reaction conditions were investigated in detail with acetic, stearic, benzoic and citric acids, and sensitive methods for the detection and determination of carboxylic acids were established. The limits of detection and the absorption maxima of the alkaline colors obtained with many carboxylic acids were determined. Most amino acids, together with hydroxybenzoic acids and aminobenzoic acids, were detected with reduced sensitivity by this method. Salicylic acid was shown by high-performance liquid chromatography and gas chromatography-mass fragmentography to form mainly its ehyl ester in the coupling reaction, whereas benzoic acid, which could be sensitively detected, formed only a small amount of the ester. p-Aminobenzoic acid also formed a large amount of its ethyl ester.