AbstractMatrix‐assisted laser desorption time‐of flight mass spectrometry has been used to investigate the affinity of aqueous copper ions for a 26‐residue synthetic peptide predicted to have metal‐binding properties. A specific reaction occurred upon the dirct application of aqueous CuSO4to the matrix/peptide sample remaining on the probe tip ater initial mass spectrometric investigation. Results indicated the binding of up to one copper ion per histidine residue and the loss of one proton for each bound copper. Bound copper was not returned to solution upon subsequent cold water washings of the probe tip. These results suggest that covalent interations, between the copper and the peptide, occurred during the probe‐tip application of the metal ion to t