Low-temperature pyrolysis of methionine-enkephalin-Arg-Gly-Leu has been carried out and the non-volatile residues have been analyzed. The fragments were separated and characterized by LC–UV/Vis–MS/MS. Two major types of pyrolysis products were identified by matching the experimental results with a theoretical list that contains the expected fragments. These products were mainly composed of cyclic oligopeptides and linear fragments produced from the peptide backbone. These fragments have preserved the sequence of amino acids in the peptide. In some cases, a complete or partial loss of an amino-acid side group was observed. Tandem mass spectrometry and cyanogen bromide cleavage experiments were used to confirm the nature of the cyclic and linear pyrolysates, in addition to chromatographic and mass spectrometric data of actual standard synthetic cyclic peptides. Copyright © 2010 John Wiley & Sons, Ltd.