Ionic liquids (ILs) are being studied as novel solvents for the sustainable extraction and separation of metal ions. Ionic liquids are solvents that consist entirely of ions. Typically, they are organic salts with a low melting point (< 100 °C). Ionic liquids have several properties that make them attractive as potential solvents for improved separation processes: wide liquidus ranges, high thermal stabilities, a negligible vapour pressure (and thus a very low volatility) and the ability to solubilize a wide range of solutes, including metal salts and complexes. A considerable variation is possible in both the cationic and anionic part of the ionic liquid. Typical organic cations are 1-alkyl-3-methylimidazolium (abbreviated as [Cnmim]+), N-alkylpyridinium, N,N-dialkylpyrrolidinium, tetraalkylammonium and tetraalkylphosphonium. Hexafluorophosphate (PF6-), and bis(trifluoromethylsulfonyl)imide (bistriflimide, Tf2N- or (CF3SO2)2N-) are used as anions in water-immiscible ionic liquids. The main rationale for using ionic liquids as the organic phase in liquid-liquid extraction processes is their low volatility and the low flammability. The replacement of organic diluents in liquid-liquid extractions by ionic liquids could lead to more sustainable extraction processes. Ionic liquids are also of interest for the processing of spent nuclear fuel. Studies showed that 1,3-dialkylimidazolium ionic liquids with chloride and nitrate anions are relatively radiation resistant and do not undergo significant decomposition by radiolysis upon exposure to high radiation doses. The stability of the ionic liquids against high radiation doses is comparable to that of benzene, but is much higher than that of the TBP/kerosene mixtures used in the PUREX process. The relatively high radiation resistance of imidazolium ionic liquids can be attributed to the presence of the aromatic imidazolium ring. Aromatic compounds have a higher stability against irradiation than non-aromatic compounds, because the aromatic ring can absorb radiation energy and can relax non-dissociatively. Moreover, mixtures of aromatic and non-aromatic compounds undergo less radiolytic decomposition than what is expected on the basis of the concentration of the non-aromatic compound, because of energy transfer to the aromatic compound. Analysis of the radiolysis products of 1,3-dialkylimidazolium chloride and nitrate ionic liquids show that the ionic liquids behave like a combination of an aromatic compound, an alkane and a salt.