The gas-phase deprotonation reactions of multiply protonated bovine ubiquitin, insulin chain B, and renin substrate tetradecapeptide ions have been studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer coupled with an external electrospray source. Rate constants were measured for the reactions of these peptide ions with a series of reference compounds of known gas-phase basicities ranging from 195.6 kcal/mol to 232.6 kcal/mol. The apparent gas-phase acidities (GAapp) of the multiply protonated peptide ions, [M+nH]n+, were determined with deprotonation reactions. The deduced values of GAapp show a strong dependence on the charge states of the multiply protonated peptide ions. In general, the values decrease as the charge states of the peptide ions increase. For ubiquitin ions, the determined GAapp values decrease from >232.6 kcal/mol to 205.0 kcal/mol for n=4-13; for insulin B ions, the GAapps decrease from >232.6 kcal/mol to 198.2 kcal/mol for n=2-5; while for renin substrate ions, the GAapps decrease from 221.6 kcal/mol to <195.6 kcal/mol for n=2-4. Surprisingly, at a given mass-to-charge ration (m/z), the GAapps of these peptide ions agree within 10 kcal/mol despite large differences in their mass and charge. Interestingly, the ubiquitin and insulin B ions generated under the present conditions reveal multiple isomers at certain charge states, n=4, 5, 6, and 12 for ubiquitin and n=4, 5 for insulin B, as evidenced by the fact that the isomers display distinctively different deprotonation reaction rates with certain reference compounds.