The synergistic ionic-liquid extraction and extraction equilibrium of lanthanoid(III) (Ln(III)) ions have been investigated using 2-thenoyltrifluoroacetone (Htta) and trioctylphosphine oxide (TOPO) in the ionic liquid 1-butyl-3 methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(4)mim][Tf2N]). The selective synergistic effect for heavier Ln(III) ions was found using a combination of Htta and TOPO in [C(4)mim] [Tf2N], leading to enhanced separability among Ln(III) ions. The extracted Ln(III) species and the extraction constants in the Htta-TOPO system were determined by three-dimensional extraction equilibrium analysis. In this system, the neutral adduct, Ln(tta)(3)(TOPO)(2), and the cationic charged adducts, Ln(tta)(2)(TOPO)(2)(+), Ln(tta)(2)(TOPO)(3)(+), and Ln(tta)(TOPO)(3)(2+), were extracted into the [C(4)mim][Tf2N] phase. The adduct formation constants in [C(4)mim][Tf2N] were determined to evaluate the synergistic effect of Htta and TOPO on the extraction of Ln(III) ions. The selective synergism for heavier Ln(III) ions in the Htta-TOPO system was ascribed to the formation of hydrophobic, charged adducts, such as Ln(tta)(2)(TOPO)(3)(+) and Ln(tta)(TOPO)(3)(2+), in [C(4)mim][Tf2N].