Aqueous two-phase systems were formed from solutions of a polyelectrolyte, polyethyleneoxide-maleic acid copolymer, and potassium phosphate. The properties of such aqueous two-phase systems were highly dependent on pH. This was reflected in the partition behavior of three chicken egg white proteins: lysozyme, conalbumin and ovalbumin. Separability of these three proteins was improved by the use of the polyelectrolyte, in comparison with when uncharged polyethylene glycol (poly(1,2-dihydroxyethane)) was used as a phase-forming polymer.