The weak intermacromolecular interactions caused by the presence of a complexing agent in a two-phase biopolymer mixture can affect its phase equilibrium and morphology. In this communication, the attempt was made to induce demixing in semidilute and highly compatible sodium caseinate/sodium alginate system (SC-SA) mixtures in the presence of a sodium salt of dextran sulfate (DSS) at pH 7.0 (above the isoelectrical point of caseins), and to characterize phase equilibrium, intermacromolecular interactions, and structure of such systems by rheo-small angle light scattering (SALS), optical microscopy (OM), phase analysis, dynamic light scattering (DLS), fast protein liquid chromatography (FPLC), ESEM, and rheology. Addition of dextran sulfate sodium salt (DSS) to the semidilute single-phase SC-SA system, even in trace concentrations (10(-3) wt %), leads to segregative liquid-liquid phase separation and a substantial increase in storage and loss moduli of the system. The degree of the protein conversion in the complex grows when the concentration of SC in the system increases from 1 to 2 wt %. It is also established here that demixing of semidilute biopolymer mixtures, induced by the minor presence of DSS, is a rather common phenomenon, because it also was observed here for other biopolymer pairs. At high shear rates SC becomes even less compatible with SA in the presence of DSS than at rest. Experimental observations suggest that the approach for inducing demixing of semidilute and highly compatible biopolymer mixtures by physical interactions of the constituents is a promising tool for regulation of biopolymer compatibility and achieving better predictions of phase behavior of aqueous protein-charged polysaccharide systems.