A protein microsequencing method for use in conjunction with the thiobenzoylation degradation of polypeptides and proteins is disclosed. The process involves reaction of the N-terminal amino acid of a polypeptide with an excess of a thiobenzoylating reagent. The derivatized polypeptide is subjected to cleavage by acid which forms a 4-substituted 2-phenyl-5(4H) thiazolone. The thiazolone is acylated to form a 5-acyloxy-2-phenylthiazole and subjected to detection by both gas chromatography and chemical ionization mass spectroscopy.