The synergistic extraction of rare-earth metals by a beta-diketone and an organophosphine oxide is described. A combination of 2-thenoyltri'uor oacetone (HTTA) and tri-n-octyl phosphine oxide (TOPO) was found to be e degrees ective in synergistically extracting rare-earth metals into an ionic liquid (IL). The synergistic e degrees ect was strongly in(similar to)uenc ed by the diluent in which the extractants were dissolved. Extraction e ciencies for all the rare-earth metal ions were enhanced by synergy when a conventional organic solvent was used (n-dodecane). However, the synergistic e degrees ect in the IL preferentially improved the scandium extraction, leading to higher selectivity for Sc than for other rare-earth metals. Slope analysis was used to assess each extraction system, and the IL itself was found to be directly involved in the complex formation of the metal extraction system.