Abstract: It is well known that in the Purex process the gamma radiolysis of the extractant TBP is a disadvantage. In the literature 2 it is reported that diphenyl sulfoxide has two-fold protective effect to inhibit V-radiolysis of TBP. In this paper the binary system of TBP-(C6H5)2SO was selected and the synergic extraction of uranyl nitrate was studied. The formation of a synergic extracting complex UO2(NO3)2·TBP (C6H5)2SO has been confirmed by the usual slope method. The extraction mechanism of uranyl nitrate with TBP or (C6H5)2 SO has been investigated respectively. There are three kinds of equilibria in the system UO2(NO3)2-TBP-(C6H5)2SO.The equilibrium constants were determined and found to be Iogβ10 = 1.44, logβ20 = -0.214 and logβ12 =0.446. By determining the effect of temperature on the distribution ratio, △H, △G, and △S of the individual reactions were evaluated as follows: for the first reaction△H=-6.31 kcal·mol-1, △G295k= -1.95 kcal·mol-1, △S298k= -14.6 cal·mol-1·K-1; for the second reaction△H=-6.86 kcal·mol-1, △G295k= -0.27 kcal·mor-1, △S298k= -23.9 cal·mol-1·K-1.The radiation stability of the system TBP-(C6H5)2 SO has been studied. The G values determined show that diphenyl sulfoxide can inhibit the radiolysis of TBP. The values of the Hafnium number are nearly the same as the pure TBP system.