Phase separation in aqueous mixtures of guar gum and casein, containing micellar casein, sodium casemate or its beta and k-fractions was investigated, and the effects of the state of casein [colloidal-dispersed (similar to 0.3 mu m), molecular-dispersed associated, and dissociated] were established by the determination of the phase diagrams. Phase separation occurring in moderately concentrated mixtures was depended on ionic strength and was not depended on the state of protein. Passing from dissociated to molecular-dispersed associated and then to colloidal-dispersed casein result in a decrease of the total concentration in the threshold point (C*(1)) in accordance to C*(1) proportional to M-W(0.27), and in increasing asymmetry of phase diagrams. In the two-phase region, the degree of compatibility was dependent on pH and ionic strength of mixtures, varying according to the change of casein-solvent interaction.