The phenylthiocarbamyl derivatives are obtained by reaction of the amino acids with phenylisothiocyanate, the reaction being essentially complete in a few minutes at ambient temperature. Phenylthiocarbamyl amino acids have been separated on Pico-Tag columns, the solvent system consisting of two eluents, an aqueous buffer and 60% acetonitrile in water. Quantitation of phenylthiocarbamyl amino acids is simplified by the finding that with the exception of lysine, all the derivatives have nearly identical molar extinction coefficients. Using the same gradient program, variation of the triethylamine concentration within relatively narrow limits affected column selectivity in a predictable manner and provided a rational basis for optimizing chromatographic conditions. After the reagent is removed from the reaction mixture under vacuum, the derivatized amino acids can be stored dry and frozen for several weeks without appreciable degradation.