Various esters and salts of N-nitrososarcosine and N-nitroso-L-proline have been prepared, and used to provide amino and carboxyl components in model peptide syntheses. The sequential polypeptide (Sar-Gly)n was synthesized using the nitroso residue as an acid-sensitive protecting group. ��� Anomalous behaviour of some of the N-nitroso derivatives in the Liebermann colour reaction is also discussed.