Proteins and polysaccharides, being the main constructional materials in many biological structures, have a limited compatibility in aqueous media. At sufficiently high concentrations, they form water-in-water emulsions. Interfacial tension is an important parameter in such systems since it is a controlling factor in the morphology development during processing. In this work a rheo-optical methodology, based on the analysis of small angle light scattering (SALS) patterns during fibril break-up, is used to study the interfacial tension of water-sodium caseinate-sodium alginate systems located close to and relatively far from the binodal. The interfacial tension close to the critical point was approximately 10(-8) N/m, and it increased considerably, to a value of up to 5.2 x 10(-6) N/m farther from the critical point. For the scaling of the interfacial tension with the density difference between the phases, a scaling exponent of 3.1 +/- 0.3 was found, in agreement with the critical mean-field scaling exponent of 3.