[a(3) + H](2+) ions generated from Ln(3+)/tripeptide complexes, where Ln = La or Ce, have similar structures to the linear [a(n)](+) ions but with protonation at both the terminal NH2 and N=CH2 groups. Ion stability is favored by having the basic secondary amine of the proline residue at the N-terminus and by an amino acid residue accommodating one of the protons on the side chain. Dissociation of [a(3) + H](2+) ions derived from peptides containing only aliphatic residues is by cleavage of the second amide bond to give [b(2)](+) or [a(2)](+) ions along with internal [a(1)](+) ions. For [a(3) + H](2+) ions containing a tryptophan residue in the central location, in addition to cleavage of the amide bond, losses of neutrals NH3, HN=CHR, (NH3 + CO), and HNCO were observed. Dissociations of some unsolvated Ln(3+)/tripeptide complexes gave [b(3) + H](2+) ions in low abundance; formation of these [b(3) + H](2+) ions was favored by the presence of a proline residue at the N-terminus and by either a histidine or tryptophan residue in the central position. Dissociation of these [b(3) + H](2+) ions was by the loss of (H2O + CO) and not only CO, indicating that these ions did not have the same type of oxazolone structure as found for [b(n)](+) ions. Density functional theory calculations suggest that the observed [b(3) + H](2+) ions of ProGlyGly were formed from [Ce(ProGlyGly)](3+) complexes in which the peptide was bound to the metal ion as an enolate. Dissociation of the slightly lower-energy complex, where the peptide is bound in the keto form, would produce an oxazolone but the high barrier required to create this isomer of the [b(3) + H](2+) ion would be sufficient to result in further dissociation. Two isomers of the [b(3) + H](2+) ion of ProHisGly have been created, one from the [Ce(ProHisGly)](3+) complex that characteristically dissociates by the combined loss of (H2O + CO) and the other by the loss of glycine from [ProHisGlyGly + 2H](2+). The [b(3) + H](2+) ion derived from [ProHisGlyGly + 2H](2+) dissociated by the loss of only CO.