The chelation of zinc by consensus zinc-finger arrays of the CCCC, CCHC, and CCHH type has been investigated by electrospray ionization mass spectrometry. Accurate mass measurements of the most abundant isotopic species have demonstrated that two protons are lost for each Zn(II) ion chelated. Methylation of zinc-finger peptides has revealed that two thiolate anions from cysteine side-chains are necessary to maintain chelation. The other cysteine(s) retain the thiol proton(s) and can be methylated without loss of chelating ability.