A series of nine tripeptide ligands of the type XCH (X = glycine or lysine, C = cysteine. H = histidine) was prepared, and the coordination chemistry of these peptides with Ni(II) was investigated. The cysteine residues were incorporated as free thiols, as protected (tert-butyl) thiols, or as disulfide-bridged cystine dimers, and the histidine residues had either carboxylate (CO2H) or carboxamide (CONH2) C-termini. The Ni(II) complexes of the protected thiols exhibited no interaction of the side chain with the metal, giving UV and electrochemical data which were consistent with related tripeptide species. The Ni(II) complexes of the free thiol-containing ligands GCH-CONH2, KCH-CONH2, and GCH-CO2H were found to dimerize rapidly via disulfide bond formation in the presence of air at pH 7. These processes were confirmed by independent synthesis of the dimeric (cystine) ligands and preparation of their Ni(LI) complexes. The disulfide-bridged complex with a carboxylate terminus Ni-2(GCH-CO2H)(2) showed no further reactivity with oxygen, which was unusual, since Ni(II) complexes of XXH-CO2H peptides are known to spontaneously decarboxylate in air.