Abstract: The extraction of uranium(Ⅵ) with petroleum sulfoxides(PSO) in different diluents is studied. The extraction ability of U(Ⅵ) decreases in the following order: benzene, toluent, cyclohexane, heptane, kerosene, carbon tetrachloride and chloroform. The influence of the concentrations of nitric acid, PSO, saltingout agent, complexing anion and temperature on the extraction equilibrium is also investigated, and the enthalpy of the extraction reaction is obtained. The relationship between the extraction equilibrium constants K ex and the physical parameters of diluents is derived. The extraction mechanism and equilibrium are examined by measurement of IR spectrophotometry.