Kinetics of the β-elimination of the phosphate group from H–Tyr–Ser(PO₃H₂)–Phe–OH and H–Tyr–Thr(PO₃H₂)–Phe–OH and subsequent addition of thiols and amines to the dehydroalaninyl and β-methyldehydroalaninyl residues formed, were followed by RP HPLC under alkaline conditions in the absence and presence of Ba²⁺ ions. By this reaction sequence, the phosphoserinyl peptide was conjugated with mono-N-(2-mercaptoethyl)amide of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (4), a mercapto-functionalized pentapeptide, H–His–Gly–Gly–His–Gly–NH(CH₂)₄SH, and an amino-functionalized fluorescent dye, 5-dimethylaminonaphthalene-1-[N-(5-aminopentyl)]sulfonamide (dansyl cadaverine). The β-methyldehydroalanine residue was, in turn, observed to be a poor Michael acceptor.