Formation of a third organic phase at high metal loading in the extraction of tetravalent actinides by TBP in aliphatic diluents has been investigated mostly from the standpoint of the composition of the organic phase species before and after phase splitting. Very little is known of the structure and morphology of the organic phase species. In this work, a study of third phase formation upon either dissolution of Th(NO3)(4) in 20% TBP in n-octane or Th(NO3)(4) extraction from 1 M HNO3 by 20% TBP in n-octane is reported. Chemical analyses have shown that, under the conditions of this work, Th(IV) exists in the organic phase mainly as the trisolvate Th(NO3)(4).(TBP)(3). The third phase species also contains a small amount of HNO3, presumably hydrogen-bonded to the trisolvate complex. Small-angle neutron scattering measurements on TBP solutions loaded with only HNO3 or with increasing amounts of Th(IV) revealed the presence, before phase splitting, of large ellipsoidal aggregates with the parallel and perpendicular axes having lengths up to about 230 and 24 A, respectively. Although the formation of these aggregates is observed in all cases, that is, when only HNO3, only Th(NO3)(4), or both HNO3 and Th(NO3)(4) are extracted by the TBP solution, the size of the aggregates is largest in the latter case. Formation of these aggregates is probably the main reason for phase splitting.