Cation radicals formed via gas-phase electron transfer to multiply protonated polypeptides have been found to react with molecular oxygen. Such cation radicals are of interest within the context of electron transfer dissociation, a phenomenon with high utility for the characterization of peptide and protein primary structures. Most of the cation radicals show the attachment of O-2 under room temperature storage conditions in an electrodynamic ion trap. At higher temperatures and under conditions of collisional activation, the oxygen adduct species lose O-2, HO circle, or HO2 circle, depending upon the identity of the side chain at the radical site. The fragments containing the C-terminus, the so-called z-ions, which are predominantly radical species, engage in reactions with molecular oxygen. This allows for the facile distinction between z-ions and their complementary even-electron c-ion counterparts. Such a capability has utility in protein identification and characterization via mass spectrometry. Intact electron transfer products also show oxygen attachment. Subsequent activation of such adducts show dissociation behavior very similar to that noted for z-ion adducts. These observations indicate that ion/radical reactions can be used to probe the locations of radical sites in the undissociated electron transfer products as well as distinguish between c- and z-type ions.