AbstractTwo methionine‐specific sequence ions are described that are formed upon the dissociation of protonated methionine‐containing peptides at high (keV) collision energies. These ions, hiherto unassigned, are denoted an+ 1 − CH3S⋅ and zn+ 1 − CH3S⋅. Precursor ion scans reveal that these species originate from the an+ 1 and zn+ 1 ions, respectively, where methionine resides at positionn‐1 relative to the N‐ or C‐terminus. A comparison of the collision‐induced dissociation (CID) spectra for several related peptides demonstrates that methionine residues are involved in the genesis of these species and mechanisms for their formation are proposed. Several structural features of a peptide that are important to the formation of these species are described. The importance of identifying these ions in order to correctly sequence a methionine‐containing peptide based on its CID sp