Polymer–ionic liquid aqueous biphasic systems (polymer–IL ABS) can be formed by the combination of distinct pairs of polymers and ionic liquids (ILs) in aqueous media. These ABS have a series of advantages relative to the conventional polymer–polymer or polymer–salt ABS. Depending on the structural features of polymers and ILs, both species seem to display the ability to act as salting-out agents. Based on a compilation and analysis of the data hitherto reported, the main issues which govern the phase behaviour of these systems are here discussed. In this respect, the effects of the chemical structure and molecular weight of polymer and IL and temperature on the liquid–liquid equilibria are addressed. The molecularlevel mechanisms behind the formation of these ABS are further highlighted and discussed based on liquid–liquid and vapour–liquid equilibria behaviour of ternary polymer–IL–water systems. It is shown that the salting-in/salting-out effects in these systems are controlled by a complex interplay of polymer–IL, polymer–water and IL–water interactions.
