The stability constants of ternary Co(II), Ni(II), Zn(II), or Cd(II) complexes containing 2,2′-bipyridyl (bipy) or 1,10-phenanthroline (phen) as a primary ligand and N-phenyl-2-mercaptopropion-amide (PMP) as a secondary ligand in a 1:1:1 molar ratio were determined using the Irving-Rossotti pH-titration technique in a 70% (v/v) dioxane-water mixture (I=0.1 M NaC104; 30 ± 1°C) in an inert atmosphere. The stability of these ternary complexes is discussed on the basis of π -accepting qualities of the sulfur ligand and the participating heteroaromatic N-base.