The loading of a 30 vol.%solution of tributyl phosphate in do-decane with large amounts of thorium(IV), uranium(IV) or pluto-nium(IV) nitrate in the presence of uranyl nitrate causes the formation of a second organic phase, which contains both the acti-nide(IV)and uranium(VI). The presence of uranium(VI) in the third phase is due to physical distribution of uranyl nitrate, solvated by tributyl phosphate, between the two organic phases. Equations are suggested which describe the distribution of some components between the organic phases, and the actinide(IV)to uranium(VI)ratio in the second organic phase. The maximum attainable concentration of actinides(IV)in the organic phase of a two-'liquid-phase system, at which a third phase is not yet formed, was measured and described by an empirical equation as a function of the concentration of uranium(VI) in the organic phase and of nitric acid in the aqueous phase.