Remarkable phenomenon that the extraction of lanthanide(III) (Ln) with thenoyltnfluoroacetone (Htta) in benzene is highly enhanced by the addition of inert mer-tris(8-quinolinolato)cobalt(III) (Co(q)3) has been found out. The effect of (Co(q)3) was ascribed to the formation of a 1:1 complex of Ln(tta)3with Co(q)3, i.e., a binuclear complex, in the organic phase on the basis of the extraction-equilibrium analysis. The formation constants determined for La(tta)3. Eu(tta)3and Lu(tta)3with Co(q)3were as high as 104.96104.56and 104.97respectively. Furthermore, in the spectrophotometric study, not only the hypsochromic shift of the ligand absorption band of Co(q)3but an isosbestic point were observed by increasing the concentration of Ln(tta)3in benzene. The formation constants of the binuclear complexes were also determined spectrophotometrically and confirmed to be comparable with those obtained by the extraction method.