The stability constants of L-norleucine (Nle) and nicotinic acid (NA) with divalent metal ion (Cu(2+), Ni(2+), and Co(2+)) binary and mixed ligand systems at 298 K and ionic strength 0.15 mol.dm(-3) NaNO(3) water solutions were determined using pH-potentiometry. The stability constants data were estimated using the HYPERQUAD 2008 program, and each complex species was shown graphically using the HySS program. UV-visible spectrophotometric measurements were performed to prove the complex species formation. In addition, the possible binding sites of the ligands Nle and NA to bind metal ions and the Gibbs free energy of some complex species were predicted and evaluated by using the Gaussian 09 program.