In 1896, Beijerinck first noted an incompatibility in solutions of agar, a water-soluble polymer, with soluble starch or gelatine.  Upon mixing, they separated into two immiscible phases.
Subsequent investigation led to the determination of many other aqueous biphasic systems, of which the polyethylene glycol (PEG) - dextran system is the most extensively studied.  Other systems that form aqueous biphases are: PEG - sodium carbonate or PEG and phosphates, citrates or sulfates.  Aqueous biphasic systems are used during downstream processing mainly in biotechnological and chemical industries.