Abstract: The present study is a detailed investigation of the synergistic extraction of uranium, thorium, neodymium and yttrium with binary system of 1-phenyl -3-methyl-4-benzoyl-pyrazolone-5(PMBP) and trioctyl-phosphine oxide (TOPO) by using a direct calorimetric titration method, together with Karl Fischer titration and solvent extraction.Calorimetric titration and temperature dependence studies of extraction method allow calculation of the enthalpy and entropy changes associated with the synergic reaction. From the magnitude and signs of the △S values, it is possible to decide whether synergism is associated with replacement of solvate water by adduct molecule TOPO with no change in coordination number or an expansion of the coordination sphere upon addition of adduct molecule but no replacement of water Uranium, thorium, neodymium and yttrium have been shown to form UO2(PMBP)2(TOPO), Th(PMBP)4 (TOPO) , Nd(PMBP)3(TOPO) and Y(PMBP)3(TOPO) respectively .Water analysis is made by Karl Fischer titration. The equilibrium constants, enthalpy,entropy and free energy changes for the addition reaction are determined. The data indicate that the main driving force of the synergistic effect is the substitution of coordinated water molecule by TOPO.