BACKGROUND: Ionic liquids (ILs) as environmentally benign solvents have been widely studied in the application of solvent extraction. However, few applications have been successfully industrialized because of the difficult stripping of metal ions or the loss of components of the ILs. More work needs to be done to investigate the extraction behaviour of IL-based extraction systems. In this work, the extraction behaviour of Ce(IV), Th(IV) and some trivalent rare earth (RE) nitrates by di(2-ethylhexyl) 2-ethylhexylphosphonate (DEHEHP) in the IL, 1-methyl-3-octylimidazolium hexafluorophosphate ([C(8)mim]PF6), was investigated and compared with that in the n-heptane system. In particular, the effect of F(I) on the extraction mechanism for Ce(IV) and its separation from Th(IV) was investigated. Otherwise, the recovery efficiency of Ce(IV) and F(I) from a practical bastnasite leach liquor was examined using IL based extraction. RESULTS: Similar selectivity (Ce(IV) > Th(IV) >> RE(III)) was observed in both the IL and n-heptane systems. The existence of F(I) had a large negative impact on the extraction and separation of Ce(IV). An identical process for the extraction of Ce(NO3)(4) containing F(I) by DEHEHP in both IL and n-heptane was achieved under appropriate conditions. The Ce(IV) in the IL phase may be quantitatively back extracted and may be recovered as part of high purity CeF3 and cerium sulfate. CONCLUSION: It is possible for IL to act as an environmentally benign solvent for the recovery of Ce(IV) and F(I) from bastnasite in rare earth hydrometallurgy. (C) 2009 Society of Chemical Industry