Collision cross section measurements and molecular modeling calculations have been combined to extract average volumes of amino acid residues in,gas-phase peptide ions. The approach uses cross sections for a series of 103 lysine-terminated peptide ions having the general form [Xxx(n)Lys+H](+) (where Xxx is any amino acid except Lys, Arg, His, and Cys, and n = 4 to 8). The results show that average volumes of individual Xxx residues in the gas phase are 5 to 15% smaller than when found in protein interiors; the Lys residue is 20% smaller in the gas phase than in protein cores. The relatively small volumes of amino acids in the gas phase compared with protein core volumes are explained by considering that few gaps or crevices along interior regions exist in compact model structures that represent the experimental data. The very small volume of the Lys residue in peptide ions is consistent with the expectation that this amino acid is the charge carrier in most (if not all) of the peptides and is subject to "electroconstriction". Some implications of these results in understanding amino acid packing in gas-phase peptide ions and in protein cores are discussed.