The separation of proteins by polyelectrolyte coacervation or precipitation is based on electrostatically-driven complex formation. We have investigated complexation between the globular protein BSA and the polyelectrolyte poly(dimethyldiallylammonium chloride) (PDMDAAC) using light-scattering techniques to monitor solution turbidity. Turbidimetric pH titrations were used to determine the specific pH values where soluble complex formation is initiated (pH(c)) and where phase separation occurs (pH(phi)). These values, collected at different ionic strengths, can be presented as phase boundaries. The effects of macromolecular concentration, protein:polymer concentration ratio (r), and polymer molecular weight upon the phase boundary are examined. The macromolecular concentration and polymer molecular weight have little or no effect on the phase boundary. While pH(c), is independent of r, pH(phi) varies inversely with r. The use of phase boundaries in the selection of optimal pH and ionic strength for maximum yield and purity in protein separations is discussed.