Recent developments in an advanced liquid-liquid extraction system for the separation of metal ions by combining a modification or chemical reaction of the metal species in the aqueous and organic phases demonstrate considerable potential. Possible techniques for the chemical conversion of aqueous-phase metal species include redox reactions for the metal ions, masking effects through the addition of water-soluble complexing agents, and complexing reactions with salting-out agents. For the conversion of organic-phase metal species, a synergistic effects through the addition of additional extractants and redox reactions for the extracted species are also useful. The separation of metal ions is effectively improved by the introduction of such chemical reactions to the extraction system.