Abstract Laser-desorbed peptide neutral molecules were allowed to react with Fe + in a Fourier transform mass spectrometer, using the technique of laser desorption/chemical ionization. The Fe + ions are formed by laser ablation of a steel target, as well as by dissociative charge-exchange ionization of ferrocene with Ne +. Prior to reaction with laser-desorbed peptide molecules, Fe + ions undergo 20–100 thermalizing collisions with xenon to reduce the population of excited-state metal ion species. The Fe + ions that have not experienced thermalizing collisions undergo charge exchange with peptide molecules. Iron ions that undergo thermalizing collisions before they are allowed to react with peptides are found to undergo charge exchange and to form adduct species [M + Fe +] and fragment ions that result from the loss of small, stable molecules, such as H 2O, CO, and CO 2, from the metal ion-peptide complex.