Abstract: A direct calorimetric investigation of the system consisting of U, Th and Gd complexes with 1-phenyl 3-methyl 4-benzoyl pyrazolone-5 (PMBP) and trioctylphosphine oxide (TOPO) in nitrobenzene was studied. In determining the heat of reaction, 55-60 ml of freshly prepared nitrobenzene solution of 150-300 mg metallic complex is introduced into the reaction vessel. When thermal equilibrium is obtained, a 0.05 M TOPO solution in nitrobenzene is delivered from the autoburette. The data points and the figures of titration curve were recorded by the computer automatically.From the experiment data, the addition of TOPO (L) to a nitrobenzene solution of UO2 (PMBP)2 and Th (PMBP)4 will form one binary synergic adducts MA2L and MA4L respectively. Gd (PMBP)3 will lead to the formation of two mixed-ligand adducts according to the equilibriawith β1, and β2 as the overall equilibrium constants and the corresponding enthalpy changes are △H1 and △H2. An iterative technique of least-square analysis was used, minimizing the error square sum by a computer program provided by Dr. M. S. Caceci. Experimental results were shown as follows: UO2 (PMBP)2-TOPO-C6H5NO2 system△H=-9.23±0.14 kJ·mol-1, logβ = 5.04±0.03;△G=-28.8±0.2 kJ·mol-1, △S=66±1 J·mol-1 K-1;Th(PMBP)4-TOPO-C6H5N02 system△H=-23.8±0.4kJ·mol-1, logβ = 5.33 ±0.22, △G= -30.4+ 1.2 kJ·mol-1, △S=22±3 J·mol-1 K-1;Gd (PMBP)3-TOPO-C6H5NO2 system△H1 = 11.7±0.45 kJ·mol-1, log β1=4.99 ±0.10, △G1 =-28.5±0.57 kJ·mol-1, △S1=34±2 J·-mol-1·K-1,△H2=-3.14±0.51kJ·mol-1, logβ2 = 7.93±0. 18,log K2 =2.94±0.20 △G2= -16.8±1.0 kJ·mol-1, △S2 = 6.7±1.2 J·mol-1·K-1, where logK2, △G2 and △S2 correspond to the equilibria