The synergistic extraction of trivalent lanthanide ions (Ln(III)) with benzoylacetone (Hba) and trioctyphosphine oxide (TOPO) in an ionic liquid (IL), 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][Tf₂N]), has been investigated. The extractability of Ln(III) with Hba was significantly enhanced in the presence of TOPO. The composition and extraction constants of the extracted species for each Ln(III) in the presence or absence of TOPO were determined by 3-dimensional equilibrium analysis. It was found that all Ln(III) were extracted as cationic ternary complexes such as Ln(ba)₂(TOPO)₂⁺ or Ln(ba)(TOPO)₄²⁺ with Hba and TOPO, while Ln(III) was extracted as a neutral or cationic species such as Ln(ba)₃ or Ln(ba)₂⁺ with Hba alone. The formation constants of the cationic ternary complexes in the IL were calculated from the corresponding extraction constants obtained, indicating that Lu(ba)(TOPO)₄²⁺ is the most stable complex in the IL. Separation efficiency between different Ln(III) was also discussed on the basis of the extraction constants. The result demonstrated that the Hba-TOPO[Bmim][Tf₂N] system presented here is superior in the separation between heavier Ln(III).