The equilibrium constants for trivalent metal ions (Fe3+ and Cr3+) and divalent metal ions (Cu2+, Ni2+ and Co2+) with ligands gallic acid, L-norleucine and nicotinic acid were studied by using pH-potentiometric titration in aqueous solution at 25oC (I = 0.15M NaNO3). In order to determine the stability constant values, the protonation constants of free ligands were also determined. The complexation model for each system was established by the software program “HYPERQUAD 2008” from potentiometric titration data. UV spectrophotometric measurement was carried out to confirm the complex that formed in each solution qualitatively. The phenomenon that occurred in each system was discussed. In terms of the nature of metal ion, the trend of stability constant is Cu2+