The first synergistic extraction between quaternary phosphonium type ionic liquid extractants in toluene for rare earth was reported in this article. There were two different ion -association mechanisms in the synergistic extraction,system. The formed reversed micelles contributed to increase extractability of the synergistic extraction system to a considerable extent. On the one hand, million tons of saponification wastewater from acidic extractants may he avoided by developing the extraction system using bifunctional ionic liquid extractants. On the other hand, the novel synergistic extraction offers an effective strategy to increase the extractabilities of industrial extractants. This article reveals sustainable and efficient potentials for industrial rare earth separation. (C) 2016 American Institute of Chemical Engineers