Abstract: Mixed ligand complexes of some transition metals[ Cu(II),Ni(II), Co(II) and Zn(II)] have been investigated Potentiometrically with Pyridine-2,6-dicarboxylic acid(PDA) and 2-hydroxy benzalidine-anthranilic acid (HBAA) in aqueous solution at 298.15 K and 313.15 K. The ionic strength of 0.1 mol dm-3 was kept constant using KNO3. The stability constants of these 1:1:1 ternary complex have been evaluated by the computational methods. The relative order of stability has been observed in accordance with the increasing (φ) = charge/ radius ratio. The changes in thermodynamic parameters (ΔG0, ΔH0, ΔS0) have also been calculated under the same conditions.