In this study, we utilized gas-phase hydrogen/deuterium (H/D) exchange reactions and ion mobility-mass spectrometry (IM-MS) to examine the effect of: (i) histidine (His) amino acid position and (ii) fragment ion size on the structures of b n ( n = 4–7) ions generated from nozzle-skimmer fragmentation. Both H/D exchange patterns and semi-log temporal plots of histidine-containing b n fragment ions generated from seven isobaric model heptapeptides { i.e. , ( His )AAAAAA-NH 2 , A( His )AAAAA-NH 2 , AA( His )AAAA-NH 2 , AAA( His )AAA-NH 2 , AAAA( His )AA-NH 2 , AAAAA( His )A-NH 2 , and AAAAAA( His )-NH 2 } suggested the presence of at least two structures for all b fragment ions: “fast” and “slow” H/D-exchanging ion populations. The observed H/D exchange rate constants (for disappearance of isolated 12 C all or D 0 ) for b 4 + and b 5 + fragments were higher than those observed for b 7 + fragment ions, suggesting more compact and/or stable structures for b 7 + fragment ions. Among the studied histidine-containing b n + fragments, b 4 + and b 5 + showed the most variation in H/D exchange reactivity as a function of histidine position in the original peptide sequence. Ion mobility arrival time distributions (ATDs) for histidine-containing b 5 + fragments from AA( His )AAAA-NH 2 , AAA( His )AAA-NH 2 , and AAAA( His )AA-NH 2 showed two ion populations. H/D exchange and ion mobility results both imply the potential presence of a mixture of compact and open structures for b 5 + . ATDs for b 4 + fragments generated from A( His )AAAAA-NH 2 and AA( His )AAAA-NH 2 (compared to ATDs of the other model heptapeptides) suggest the appearance or increase in the percentage of a more compact ion population. ATDs of histidine-containing b 6 + and b 7 + fragments varied significantly as a function of histidine position in the original heptapeptides.