The stability constants of ternary Co(II) or Ni(II) complexes containing 2,2′-bipyridyl (bipy) or 1,10-phenanthroline (phen) as primary ligand and N-phenyl 2-mercaptoacid amide (PMA), N-(p-chloro)phenyl-2-mercaptoacid amide (CPMA), N-(p-toly1)-2-mercaptoacid amide (TMA) or N-(p-anisyl)-2-mercaptoacid amide (AMA) as secondary ligand were determined in 70% (v/v) dioxane-water medium at 30 ± 1°C and 0.1M(NaC1O4) ionic strength in a nitrogen atmosphere. A study Tf Co(II) or Ni(II) in the presence of an equimolar concentration of the two ligands showed the formation of mixed ligand complexes containing a 1:1:1 molar ratio of metal and the two ligands. These ternary complexes are more stable than would be expected on a statistical basis, i.e., the difference, Δ log KM= log KMAMAL- log KMML, is positive (A = bipy or phen, L = secondary ligand). Further results suggest that both the ligands are incompatible toward the metal ions and hence the mixed-ligand complexes formed are relatively more stable.