The unimolecular and low energy collision-induced fragmentation reactions of the MH(+) ions of N-acetyl-tri-alanine, N-acetyl-tri-alanine methyl ester, N-acetyl-tetra-alanine, tetra-alanine, penta-alanine, hexa-glycine, and Leu-enkephalin have been studied with a particular emphasis on the formation and fragmentation of B n (n=3,4,5) ions. In addition, the metastable ion fragmentation reactions of protonated tetra-glycine, penta-glycine, and Leu-enkephalin amide have been studied. B n ions are prominent stable species in all spectra. The B n ions fragment, in part, by elimination of CO to form A n ions; this reaction occurs on the metastable ion time scale with a substantial release of kinetic energy (T 1/2=0. 3-0. 5 eV) that indicates that a stable configuration of the B n ion fragments by way of a reacting configuration that is higher in energy than the fragmentation products, A n + CO. Ab initio calculations strongly suggest that the stable configuration of the B3 and B4 ions is a protonated oxazolone formed by interaction of the developing charge with the next-nearest carbonyl group as HX is lost from the protonated species H-(Yyy) n -X · H(+). The higher B n ions also fragment, in part, to form the next-lower B ion, presumably in its stable protonated oxazolone form. This reaction is rationalized in terms of the three-dimensional structure of the B n ions and it is proposed that the neutral eliminated is an α-lactam.