The synergistic extraction of lanthanoids(III), Ln(III),i e., La, Sm, Tb, Tm and Lu, using 2-thenoyltrifluoroacetone (Htta) and nitrogen-containing neutral bidentate ligands (S),i.e., three ethylenedi-amine derivatives, such as N,N'-dimethylethylenediamine (dmen), N,N'-diethylethylenediamine (deen) and cis-l,2-cyclohexanediamine (chda), was studied in the benzene and aqueous system. The synergistic enhancement in these extraction systems was mainly attributed to the formation of an adduct, Ln(tta)3S, in the benzene phase. The variation of the adduct formation constants (βs,l) was discussed with the basicity of ligands and structure hindrance around the metal ion.