Three aromatic isothiocyanates were surveyed as possible precolumn derivatizing agents for the electrochemical detection of amines and amino acids. From the isothiocyanates studied, p-N,N-dimethylaminophenylisothiocyanate (DMAPI) was chosen for further development as a label in the high-performance liquid chromatographic determination of amino acids. Amino acids reacted with DMAPI to form the corresponding substituted phenylthiohydantoins, which have been isolated and characterized, and which can be reversibly oxidized at a glassy carbon electrode at pH 2 with E 1 2 = 0.68 ± 0.01 V vs. Ag/AgCl. The derivatized amino acids were separated on a bonded C8 column in 0.1 M phosphate buffer-25% acetonitrile (pH 2 or 6), and detected at a glassy carbon electrode set at 0.85 V vs. Ag/AgCl. A mixture of 21 amino acids has been separated with 80-90% recovery, and with a linear response from 1 to greater than 150 ng and detection limits of 0.5 to 1 ng.