In this work, the combined applications of potentiometric, UV-vis and CD spectroscopy measurements on the Ni(II) complexes of GlyGlyHisGly and GlyGlyHisAla tetrapeptides were studied, in samples containing metal ions and ligand in equimolar ratio. The ligand deprotonation constants were calculated using SUPERQUAD software. The reversibility, other redox or hydrolictic process of these complexes were checked. The exact coordination modes and existence of other new complexes in highly alkaline solution were determined.