A highly sensitive and selective fluorimetric determination method for dicarboxylic acids (C5-C12) has been developed. This method is based on an intramolecular excimer-forming fluorescence derivatization with a pyrene reagent, 4-(1-pyrene)butyric acid hydrazide (PBH), followed by reversed-phase liquid chromatography (LC). The carboxylic acids were converted to the corresponding dipyrene-labeled derivatives by reaction with PBH in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The derivatives afforded intramolecular excimer fluorescence (450-550 nm) which can clearly be discriminated from the normal fluorescence (370-420 nm) emitted from PBH and monopyrene-labeled derivatives of monocarboxylic acids. The structures of the derivatives and the emission of excimer fluorescence were studied by LC with mass spectrometry and with spectrofluorimetry, respectively. The PBH derivatives of the carboxylic acids could be separated by reversed-phase LC on an ODS column with isocratic elution. The detection limits (signal-to-noise ratio = 3) were 1.3 fmol to undetectable for a 20-microl injection.