Although the majority of biopolymers are incompatible in water, systems containing casein molecules and a neutral polysaccharide (guar gum galactomannan) showed phase separation only at an ionic strength above 0.09-0.2. Static light scattering, circular dichroism spectroscopy, velocity sedimentation, viscosimetry, phase analysis in different solvents, and Rosenberg＇s method were used to estimate the effect of polymer-solvent and polymer-polymer interactions on the phase state of casein-guar aqueous systems. Different solvent conditions were used to try to clarify the nature (electrostatic or nonelectrostatic) of the interaction between the two macromolecular species. Data obtained show that the dominant mechanism controlling the single-phase state at low ionic strength (below 0.01) involves the formation of water-soluble weak interpolymer complexes, which may be destroyed by increasing ionic strength.