Polymer-ionic liquid aqueous biphasic systems (polymer-IL ABS) can be formed by the combination of distinct pairs of polymers and 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 IL, both species can 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 structure and molecular weight of polymer, temperature, and structure of anion and cation of IL on the liquid-liquid equilibria are addressed. The molecular-level 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/-out effects in these systems are controlled by a complex interplay of polymer-IL, polymer-water and IL-water interactions.
