Relative energetics of fragmentation of protonated peptides are investigated by using electrospray ionization/surface-induced dissociation (ESI/SID) tandem mass spectrometry. ESI/SID fragmentation efficiency curves (percent fragmentation versus laboratory collision energy) are presented for 20 oligopeptides and are a measure of how easily a peptide fragments. The relative positions of the ESI/SID fragmentation efficiency curves depend on several parameters which include peptide composition (e.g., presence/absence of a basic amino acid residue) and peptide size. The ESI/SID fragmentation efficiency curves, in combination with quantum chemical calculations, provide a unique approach to substantiate and refine the mobile proton model for peptide fragmentation. Selected peptides are also investigated to further test and confirm the mobile proton model; these include doubly-protonated peptides and chemically-modified peptides (i.e., acetylated and fixed-charge derivatized peptides). Doubly-protonated peptides fragment more easily than the singly-protonated forms of the same peptides, with a sequence dependence for the difference in energy required for the fragmentation of singly- vs doubly-protonated peptides. Acetylation at the amino terminus and arginine side chain leads to a decrease in basicity and a corresponding lower energy onset for fragmentation than for the unmodified form of the peptide. Fixing the site of charge by addition of trimethylammonium acetyl to the amino terminus, i.e., eliminating the mobile proton, results in a higher energy onset than that for the protonated form of the same peptide. Curves for doubly protonated peptides with two adjacent basic residues (Arg, Arg) suggest the localization of the two protons at the two basic side chains rather than at opposite termini of the peptide.