Two of the most widely used industrial extractants for rare earth elements (REEs), that is, di(2-ethylhexyl)phosphoric acid (HDEHP) and 2-ethyl(hexyl)phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) were developed into [DEHP](-) type acid-base coupling bifunctionalized ionic liquids (ABC-BILs) and [EHEHP](-) type ABC-BILs, respectively. The combinations of ABC-BIL extractants revealed synergistic effects for REEs. Seven different combinations of ABC-BILs and five kinds of REEs confirmed the novel synergistic extraction. Some synergy coefficients of the combined ABC-BILs were bigger than those of mixed HDEHP and HEH[EHP] by two orders of magnitude. The first synergistic extraction produced by ionic liquid extractants in the field of solvent extraction was reported in this article. The novel synergistic extraction from combined ABC-BILs extractants revealed highly efficient and environmentally friendly potential in both of academic research and industrial application for REEs separation. (C) 2014 American Institute of Chemical Engineers