We studied the stepwise formation constants (β) of water-soluble diglycolamide (DGA) and dioxaoctanediamide (DOODA) for the mutual separation of Ln in a solvent extraction system. TODGA (N,N,N',N'-tetraoctyl-diglycolamide) and DOODA(C8) (N,N,N',N'tetraoctyl-dioxaoctanediamide) exhibit opposite behaviors in extracting both light and heavy Ln through Ln-patterns. Metal complexes of two-and three-folding with watersoluble DOODA and DGA, respectively, were found, and each β value was calculated using distribution ratios. Taking β, their distribution ratio, D, and separation factor, SF, values into consideration, the suitable separation conditions (aqueous phase: 30 mM DOODA(C2) in 1 M HNO3; organic phase: 0.1 M TODGA in n-dodecane