A potentiometric titration technique has been used to determine the stability constants for the various complexes of Ni(II) with bio-molecules(drugs) (L) as primary ligand and amino acids (R) as secondary ligand. Ternary complexes of amino acids are formed in a stepwise mechanism, whereby (L) binds to Ni(II),followed by interaction with ligand (R), form ternary complexes through a simultaneous mechanism. The formation constants of the complexes were determined at 30ÃÆÃÂ¢ÃâÃ¢â¬âÃâÃÂ¦C and ionic strength 0.1M NaClO4. The relative stabilities of the ternary complexes are compared with those of the corresponding binary complexes in terms of Δlog K, KR, KL and Kr values. The concentration distribution of the complexes is evaluated.