Positive and negative secondary ion mass spectra obtained by bombardment of thin films of leucine-enkephalin and methionine-enkephalin with a pulsed beam of Cs+ ions of energy 5–15 keV were recorded using a time-of-flight mass spectrometer. Many of the secondary ions are metastable. The spectra contain a wealth of information, including the following important features: (i) ions characteristic of the intact molecule, from which the molecular weight may be deduced; (ii) ions diagnostic of the amino acid composition of the peptide; (iii) ions diagnostic of the sequence of amino acids in the peptide.