Abstract— <p>A mass spectrometric study is performed of the ionization of short peptides triglycine, alanyl glutamine and prolyl leucine via electrospray ionization (ESI) and surface-activated laser desorption/ionization (SALDI) using copper sulfate crystal hydrate. It is shown that having copper ions in a solution during ERI ionization initiates the aggregation of peptide molecules with the formation of large associates of up to 7–8&#xa0;peptide molecules. The effect the nature of peptides has on that of ionization is studied. During SALDI ionization, there is competitive cationization of peptide molecules by copper ions, accompanied by the formation of ions M + Cu⁺. The fragmentation of peptides and the copper cationization of decarboxylation products are also characteristic.</p>