The interaction of transition metal ions and rare earth metal ions with Isoniazid drug has been investigated in 20% (V/V) ethanol-water mixture at 0.1 M ionic strength at temp 298 K by Potentiometric titration .The data obtained is used to calculate the values of proton- ligand stability constant (pK) and metal-ligand stability constant (logK). It is observed that transition metal ions and rare earth metal ions forms 1:1 and 1:2 complexes.