Action spectroscopy of protonated substance P, a model undecapeptide, has been probed from 5.2 eV to 20 eV. For photon energy above the ionization threshold measured at 10.3 ± 0.1 eV, the radical dication is observed along with side chain losses and abundant formation of all kinds of sequence ions. Below the photoionization threshold, the photoproducts involve side chain cleavages and backbone cleavages into a-, b-/y-, and c-sequence ions. Different electronic excited states appear to produce different fragment ions. Norrish type I and II reactions are proposed to account for some photoproducts. This study bridges the gap between laser activation and electron-induced dissociation of peptides. Moreover, our results report for the first time a comprehensive picture of the photochemical fragmentation of a gas phase peptide in a wide photon energy range.