Liquid-liquid extraction (LLE) in systems composed of a water-rich phase and an organic-rich phase is widely used in organic chemistry and the chemical industry . However, the use of an organic extract phase is often inappropriate for solventsensitive biomolecules, such as proteins, and extractive purification of these compounds is, therefore, more easily and effectively conducted in aqueous two-phase systems. Albertsson [2, 3] pioneered aqueous two-phase extraction technology and several of the aqueous two-phase systems he discovered are still used to purify biologic products, as well as in high-resolution small scale bioassays [4, 5]. Aqueous two-phase systems are formed by adding to water two polymers at sufficiently high concentrations so that the mixing enthalpy dominates over the mixing entropy and drives phase separation. The mixing of water, a soluble polymer and a strongly dissociating salt present at high concentration can also generate aqueous two-phase systems useful for LLE. The most well-studied example of the two-polymer class of aqueous two-phase systems is the poly(ethyleneglycol) (PEG)-dextran system, while the poly(ethyleneglycol) (PEG)-phosphate system is the best studied polymer-salt type two-phase system. In these and other two-phase systems, each phase generally contains 80% (wt/wt) to 95% water. It is the high water content of each phase that makes the extraction system particularly suitable for purifying solvent-sensitive biomaterials, such as proteins, cell organelles, and whole viable cells [3, 6-8].