Mild oxidation with periodate of the 1-amino-2-ol moiety of N-terminal seryl or threonyl peptides and proteins leads to a terminal aldehyde function O=CH-CO- which usually may be exploited for bioconjugate formation (e.g., via oximation with an O-alkyl hydroxylamine). We report that, when followed by a prolyl residue, the O=CH-CO- group can undergo a rapid cyclization and dehydration reaction through nucleophilic attack by the amide nitrogen of the third amino acid residue of the chain. We have characterized the resulting heterocycle, which is stable in aqueous acid, by mass spectrometry and NMR. Quantitative oximation can nevertheless be achieved in such cases by performing a one-pot oxidation-oximation without isolation of the intermediate aldehyde, as is demonstrated with cholera toxin B subunit.