Such physicochemical properties of bis(2-ethylhexyl)phosphinic acid, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester and bis(2-ethylhexyl)phosphoric acid as the solubilities and the dissociation constants in water, the dimerization constants in organic solvent and distribution constants in organic solvent-water were examined by a potentiometric two-phase titration. The solvent extraction of seven kinds of transition metal ions and magnesium(II) from aqueous sulfate media was studied with the three organophosphorus acids. The pH1/2 values obtained were discussed in terms of the separation efficiency between nickel(II) and the other metal ions. Bis(2-ethylhexyl)phosphinic acid was found to exhibit the best separation efficiency compared with the other two extractants. As a practical application, the purification of unrefined nickel sulfate solution containing the other metal ions as impurities with three extractants was studied by the sequential multistage extraction method with a separatory funnel.