The phase behavior of ternary (protein / dextran / solvent) and quaternary (protein 1 / protein 2 / dextran / solvent) aqueous solutions have been monitored at various ionic strengths and pHs by turbidity measurements. For the ternary systems, phase separation was favored when the solvent conditions promoted protein self-association. Bovine serum albumin (BSA) and g-globulin were believed to self-associate in the presence of dextran at their respective isoelectric points through the electrostatic interactions between positive and negative charged patches at their surface. The fact that phase separation for BSA became less evident on increasing the ionic strength supported this concept. In the case of lysozyme alone, self-association was promoted by the addition of electrolyte as was phase separation in the lysozyme / dextran ternary system. For this biopolymer self-association is presumably as a consequence of van der Waals and/or hydrophobic forces. The influence of dextran has been discussed in terms of polymer depletion theory where an additional attractive force is created due to the exclusion of the dextran molecules from the immediate vicinity of the protein surface. For the quaternary systems, by subtle choice of solvent conditions, BSA and globulin could be exclusively partitioned into separate liquid layers through a segregative mechanism, while BSA and lysozyme could be separated into a single highly concentrated layer through an associative mechanism.