In spite of its technological importance, third phase formation in the extraction of hexavalent actinides from nitric acid solutions into alkane solutions of tri-n-butylphosphate (TBP) has received only limited attention. The focus of the few available literature works has been primarily centered on the composition of the third phase and on the stoichiometry of the metal complexes. Very little is known, on the other hand, about the structure and morphology of the third phase species of hexavalent actinides. In the present investigation, the formation of a third phase upon extraction of U(VI) by 20% TBP in deuterated n-dodecane from nitric acid solutions was studied. Chemical analyses have shown that U(VI) exists in the third phase as a species having the composition UO2(NO3)(2).(TBP)(2).HNO3. Small-angle neutron scattering measurements on TBP solutions loaded with only HNO3 or with increasing amounts of U(VI) have revealed the presence, both before and after phase splitting, of relatively large ellipsoidal aggregates with the parallel and perpendicular axes having lengths up to about 64 and 15 Angstrom, respectively. The formation of these aggregates is observed in all cases, that is, when only HNO3, only UO2(NO3)(2), or both HNO3 and UO2(NO3)(2) are extracted by the TBP solution. Upon third phase formation, the SANS data reveal the presence of smaller aggregates in both the heavy and light organic phase.