The conformations of cytochrome c ions (+8 through +18) in the gas phase are examined by simultaneous ion-mobility and hydrogen-deuterium exchange measurements. By varying the voltage used to inject ions into the drift tube it is possible to study H/D exchange of specific conformers observed in the ion-mobility spectra : either very diffuse structures that are favored for all charge states at high injection voltages or compact structures that can be favored for the +8, +9, and +10 charge states at low voltages. The number of exchangeable hydrogens for the diffuse conformer is independent of charge state, with an average value that is quite low : only 63 +/- 2 of a possible 198. The compact conformers observed for the +8 through +10 charge states exchange fewer hydrogens (similar to 46), consistent with the idea that compact structures protect some hydrogens in the gas phase. Many sites that rapidly exchange in solution appear to be restricted for exchange in the gas phase, even for very open conformers.