Abstract: The thermodynamics of the reaction between UO2(TTA)2 and Th(TTA)4 with N,N-dimethylheptyl-acetamide (N503) in "dry" and "wet" toluene solution at 298.15 K has been studied by titration calorimetry. The heat of the reaotion is determined by TRONAC Model 1250 automatic isoperibol calorimeter. The data points and figures of titration curve are recorded by the computer automatically.From the experimental data, the addition of N503(L) to a toluene solution of UO4(TTA)2 and Th(TTA)4 will form one binary synergic adducts MA L and MA4L respectively according to the equilibriawhere β1 is the equilibrium constant and △H1 is the corresponding enthalpy change. An iterative technique of least-square analysis is used to calculate β, and △H , minimizing the error square sum by a computer program provided by M.Caceci then improved by Yang Hua.The calculated equilibrium constants and the thermodynamic function, including the entropy and free energy changes calculated simultaneously are summarized in Table 1, with uncertainties expressed as mean deviations.