An unprecedented method of producing molecular radical cations of oligopeptides in the gas phase has been discovered. Electrospraying a methanolic mixture of a Cu(II)-amine complex, e.g., Cu-II(dien)(NO3)(2) (where dien = diethylenetriamine), and an oligopeptide (M) yields the [Cu-II(dien)M](. 2+) ion, whose collision-induced dissociation (CID) produces [Cu-I(dien)](+) and M.+, the molecular cation of the oligopeptide. Abundant M.+ is apparent when the oligopeptide contains both a tyrosyl and a basic residue-arginyl, lysyl, or histidyl. These structural requirements are similar to those in the metalloradical enzyme process in photosystem II. Tandem mass spectrometry of M.+ produces fragment ions that are both common to and also different from [M + H](+). The fragmentation chemistry of M.+ and of its products appear to be radical driven.