The extraction of tetravalent cerium, Ce(IV), from aqueous nitric acid with tri-n-butyl phosphate (TBP) in n-dodecane was studied by varying the aqueous, initial cerium(IV) concentration, [Ce4+](aq,init), up to and beyond the point of third phase formation, defined by the critical aqueous concentration (or CAC) and the limiting organic concentration (or LOC). The new chemistry, elaborated here for the nearly-century-old Ce(IV)-20% TBP system, focuses on the phenomena of third phase inversion and the distribution of four solutesCe(IV), HNO3, H2O, and TBPbetween the aqueous and organic phases, which are of direct relevance to the PUREX process. We demonstrate that multinuclear Ce(IV) entities are present in the organic phases.