Abstract: The formation of a third phase is studied in the system Pu(Ⅳ) nitrate-nitric acid-TBP-alkane diluent (or kerosine) . The maximum concentration (solubility, Spu) of Pu(Ⅳ) in the equilibrium organic phase at which still no third phase is formed is measured as a function of the nitric acid concentration in the equilibrium aqueous phase CH,the temperatu. e,and the molecular size of n-alkane diluent.Results show that maxima of SPu at 6 to 7 M nitric acid are observed at 17℃,26℃ and 35℃ while not at 40℃.Minima on the Spu vs. CH curves at 0.5 to 2 M nitric acid have been observed at 17℃ and 26℃.It is found that the molecular size of n-alkane diluents has a very pronounced effect on Spu. The effect of aqueous equilibrium Pu(Ⅳ) concentration on the formation of a third phase is investigated, when the equilibrium HNO3 concentration is 3.5 M. It is shown that the plutonium concentration of the third phase is increased with increasing aqueous equilibrium Pu (Ⅳ) conceatration.The effect of the addition of lauryl alcohol on the Spu is preliminarily studied.