We have studied the behaviour of the brain pentapeptide leucineenkephalin on phosphoric acid spiked glycerol matrices under FAB-PI conditions. Ion series of the general formula [mM+nH3PO4+H]+, [mM+H+62]+and [mM+H3PO4+H+62]+were observed. Experiments using labelled glycerol, methionine-enkephalin and various spiking agents were performed along with B/E linked scans in order to investigate the nature of the latter two series of ions; they led to the conclusion that the +62 ions originate from the slow decomposition of H3PO4-peptide aggregates.