[en] The solvent extraction of lanthanum(III), europium(III), lutetium(III), scandium(III), and indium(III) in 0.1 mol dm^-3 sodium nitrate solutions with 2-thenoyltrifluoroacetone (Htta) in the absence and presence of tetrabutylammonium ions (tba⁺) into carbon tetrachloride was measured. The extraction of lanthanum(III), europium(III), and lutetium(III) was greatly enhanced by the addition of tba⁺; this could be explained in terms of the extraction of a ternary complex, M(tta)₄^-tba⁺. However, the extractions of scandium(III) and indium(III) were nearly the same when tba⁺ was added. The data were treated on the basis of the formation equilibrium of the ternary complex from the neutral chelate, M(tta)₃, with the extracted ion-pairs of the reagents, tta^-tba⁺, in the organic phase. It was concluded that the degree of association of M(tta)₃ with the ion-pair, tta^-tba⁺, is greater in the order La(tta)₃ ≅ Eu(tta)₃ > Lu(tta)₃, or that the stability of the ternary complex in the organic phase is higher in the order La(tta)₄^-tba⁺ ≅ Eu(tta)₄^-tba⁺ > Lu(tta)₄^-tba⁺. This is similar to those of adduct metal chelates of Htta with tributylphosphate (TBP) in synergistic extraction systems. (author)