Abstract A new derivatization method for carboxylic acids in aqueous solution for analysis with capillary electrophoresis and laser-induced fluorescence detection was developed. The derivatization is based on a phase-transfer reaction with 4-aminofluorescein as fluorophore and dicyclohexylcarbodiimide as activating agent. This reaction procedure includes an enrichment factor of the analyte up to 10-fold. The limits of detection of carboxylic acids are in the range of 3–150 nmol/l. An advantage of this system is the indifference against inorganic matrices like carbonate, chloride, etc., up to the mmol/l range. Using this technique for the first time, the measurement of diurnal profiles of carboxylic acids (C 5–C 9) in ambient air with a time resolution of 1 h is possible.