Abstract: The method of two-phase titrations was first suggested by D.Dyrssen (1952) and B.S.Jensen (1959).In the present investigation we have extended the method to the study of mechanism of chelate and synergistic extraction and for the determination of stability constants of mixed ligand complexes of the following type:Mⁿ⁺+nA^- +j(HA)_(a) +kB_(o) = MA_n(HA)_j(B)_k(o) β_njk = (MA_n(HA)_j(B)_k)_o/(Mⁿ⁺)(A^-)ⁿ(HA)_o^f(B)_o^k and we have offered a new and rigorous derivation of the theoretical formulas necessary for the evaluation of β_njk, The method was applied to the study of the mechaniam of chelate and synergistic extraction of zinc ion with l-pheny-3-methyl-4-benzoyl-pyrazolone(HPZL) and TBPin benzene.The results and the scope of applicability of the two-phase titration method were discussed.