The synergic extraction of uranium(VI) from nitric acid solution with a mixture of bis(octylsulfinyl)ethane (BOSE) and petroleum sulfoxides (PSO) in 1,1,2,2- tetrachloroethane has been studied. The composition of the complex of the synergic extraction is UO2(NO3)2·BOSE·PSO. The formation constant of the complex is KPT = 4.44±0.23. The effect of extractant concentration, nitric acid concentration, salting-out agent concentration and temperature on the extraction equilibrium of uranium(VI) is also studied and the enthalpy of the extraction reaction is calculated. It has been found that the maximum synergic extraction effect occurs if the molar ratio of PSO to BOSE is one to one.