Abstract: The present invention provides methods and reagents for sequencing amino acids. One embodiment of the method for determining the terminal amino acid of a substantialy pure polypeptide comprises the steps of (a) attaching the polypeptide to a solid support, (b) reacting the polypeptide with a compound described below, under conditions and for a time sufficient for coupling to occur between the terminal amino acid of the polypeptide and the compound, thereby yielding a polypeptide with a derivatized terminal amino acid, (c) washing the solid support to remove unbound material, (d) cleaving the derivatized terminal amino acid from the polypeptide with a cleaving agent, (e) ionizing the cleaved derivatized terminal amino acid, and (f) determining the molecular weight of the derivatized terminal amino acid, such that the terminal amino acid is determined.Within one embodiment, the compound is p-isothiocyanato phenethyl trimethylammonium and counterion salts thereof.