Abstract: The stability constants of 1:1:1 mixed ligang complexes of Pb(Ⅱ) with 2,2' -dipyridyl(dipy) or 1,10-phcnanthrolinc(phcn) as the first ligand,and malo-natc (mal) as the second ligand were determined at 25±0.1℃ with ionic strength 0.10 (maintained by KNO₂) by using potcntiometric method. The △logK and logX of these mixed ligand complexes and their statistical expected values were calculated. The results showed that the above mixed ligand complexes have an enhanced stability.The reasons that lead to this results are discussed.