The stability constants of the binary complexes of type PbB, PbB_2, and the mixed ligandcomplexes of type PbAB have been studied by potentiometric pH titration technique at ionic strengthI=0.10 (KNO_3) and at temperature 15, 25, 35 and 45℃ respectively, where A=2,2’-bipyridyl(bipy) or 1, 10-phenanthroline (phen); B=malonate (mal), succinate (suc), or anthranilate (anth). Theequalibrium constantslogK,~1G,H, andS of the reaction PbA+PbB=PbAB+Pb~2+have been calculated. The results show that the discriminating effects~2 between the primary ligand(bipy or phen) and the secondary ligand (real, suc or anth) in those non-transition metal mixed ligandcomplexes are also evident, and as a measurement of this effect,H is more appropriate thanlogK.The possible reasons which lead to these results have been discussed. The stability constants of the binary complexes of type PbB, PbB_2, and the mixed ligand complexes of type PbAB have been studied by potentiometric pH titration technique at ionic strength I = 0.10 (KNO 3) and at temperature 15, 25, 35 and 45 ℃ respectively, where A = 2,2’-bipyridyl (bipy) or 1,10-phenanthroline (phen); B = malonate (mal), succinate (suc), or anthranilate (anth). Theequalibrium constantslogK, ~ 1G, H, andS of the The results show that the discriminating effects ~ 2 between the primary ligand (bipy or phen) and the secondary ligand (real, suc or anth) in those non-transition metal mixed ligand complexes are also evident, and as a measurement of this effect, H is more appropriate than logK. The possible reasons which lead to these results have been discussed.