Abstract Fragmentation of some electrospray-generated complex ions, [ 63Cu II(amine)M] ·2+, where M is an enkephalin derivative, produces the radical cation of the peptide, M ·+. This ion has only been observed when M contains a tyrosyl or tryptophanyl residue plus a basic residue, typically arginyl or lysyl. A typical viable amine is diethylenetriamine. Collision-induced dissociation (CID) of the M ·+ ion yields a prominent [M − 106] ·+ product ion for tyrosine-containing peptides, and a prominent [M − 129] ·+ ion for a tryptophan-containing peptide. These fragment ions are formed as a result of elimination of the tyrosyl and tryptophanyl side chains. Dissociation of these ions, in turn, produces second generation product ions, many of which are typically absent in the fragmentation of protonated peptide ions. Structures for some of these unusual ions are proposed.