Abstract: The influence of the temperature, as well as the concentrations of nitric acid, petroleum sulfoxides (PSO),salting-out agent and metal ions on the distribution ratio of uranium(VI),thorium(IV) has been systematically studied. It is found that the extraction regularity of PSO is similar to that of DOSO and TBP. Both uranium (VI) and thorium (IV) exibit the maximum distribution ratios at 5-6 mol/1 HNO3 and 3-4 mol/1 HNO3,respectively. The extracted compounds are determined to be UO2(NO3)2·2PSO and Th(NO3·3PSO,respectively. The distribution ratios of uranium(VI) and thorium(IV) increase rapidly in the presence of salting-out agent. The extraction enthalpies of uranium (VI) and thorium (IV) with PSO are △HU = - 26 kJ/mol and △HTh =- 29 kJ/mol.