Computer-based analysis of the pH-titration data obtained in aqueous perchlorate medium at 37 °C and I= 0.15 mol dm–3(NaClO4) for NiII–A–B ternary systems [A =L-cysteine (Cys), D-penicillamine (pen) or L-cysteic acid (Cya); B = imidazole (Him), histamine (Hist) or L-histidine (His)] showed the presence of the ternary species NiABH2, NiABH, NiAB and NiAB2 in addition to binary species with A or B. Under the same experimental conditions, binary systems of NiII with Cys, pen, Cya, Him, Hist and His have also been studied. For the NiABH species in the NiII–Cys–Him, –His and –Hist and NiII–pen–Him mixed-ligand systems, the site of protonation is on the primary ligand (A), while in the NiII–A–His and –Hist systems (A = pen or Cya) protonation occurs at the secondary ligand. In the NiABH2 species, one proton is attached to the primary ligand and the other to the secondary ligand. For the NiAB and NiAB2 species the Cys and pen primary ligands bind the metal through the mercapto and amino groups, while Cya binds NiII in a glycine-like mode. In these species, the Him, Hist and His secondary ligands are respectively mono-, bi-and ter-dentate. The probable reason for the higher stability of NiAB2 species compared to NiAB species in the NiII–A–Him systems (A = Cys, pen or Cya) is discussed.