Abstract: The present invention consists in a method of preparing amino thiohydantoins either in isolation or as the C-terminals residue of a peptide. The method comprises reacting the amino acid or peptide with an acylating agent and thiocyanate or isothiocyanates in the presence of a strong acid. The present invention also relates to an improved method for C-terminal sequencing of peptides which routinely analyses all of the common amino acids of peptides. The invention involves the use of a strong, volatile, anhydrous organic or mineral acid to cleave the terminal amino acid thiohydantoin.