<jats:title>Abstract</jats:title><jats:p>A new complex of cyclic peptide lactone antibiotics from <jats:italic>Bacillus subtilis</jats:italic>, which we named <jats:italic>Maltacines</jats:italic>, has recently been described. The structure elucidation of four of them is reported in this paper. The amino acid sequences and structures of the peptides were found by MS<jats:sup><jats:italic>n</jats:italic></jats:sup> of the ring‐opened linear peptides, which gave uninterrupted sequences of Bn and Y″n ions. The identities of three unknown residues in the sequences were solved by a combination of derivatisation with phenylisothiocyanate (PITC), high‐resolution mass spectrometry and H/D exchange. The nature and position of the cyclic structure was disclosed by a chemo‐selective ring opening with Na<jats:sup>18</jats:sup>OH and was found to be a lactone formed between a hydroxyl of residue number 4 and the <jats:italic>C</jats:italic>‐terminal amino acid number 12. For verification of the structure of the B2<jats:sup>+</jats:sup> ion, peptides with different combinations of P/Q and P/K at the <jats:italic>N</jats:italic>‐terminus were synthesised. The structure of the four peptides were found to be: C1a and C2a: cyclo‐4,12(P‐Q‐Y‐Adip‐V‐E‐T‐Y‐Orn‐103‐Y‐I‐OH) and C1b/C2b: cyclo‐4,12(P‐Q‐Y‐Adip‐V‐E‐T‐Y‐K‐103‐Y‐I‐OH). Adip = aminodihydroxy pentanoic acid. Copyright © 2005 John Wiley & Sons, Ltd.