Back-extraction of Pu4+ from a mixture of 20% tributyl phosphate (TBP) and 20% mixed trialkyl phosphine oxides (TRPO) in kerosene in the presence of UO22+ was studied. The back-extractants investigated may be divided into three groups: carboxylic acids and salts, amino polycarboxylates, and phosphonic acid. The distribution coefficients of both Pu4+ and UO22+ using a number of different back-extractants were measured and compared. The results obtained suggest that the only practical back-extractants are carboxylic acids. Among the carboxylic acids tested, oxalic acid is suitable when the UO22+ concentration in the organic phase is less than 2 g/L. For UO22+ concentrations between 2 and 10 g/L, oxalic acid-nitric acid mixtures may be used. For UO22+ concentrations greater than 10 g/L, the only practical back-extractant is glycolic acid. The results obtained here may be used to further develop a new process for separation of Pu4+ and UO22+ from TBP-TRPO/kerosene mixture.