Polymer interactions are used in a variety of fields (i.e.,medical, biochemical, food science, and polymer chemistry). Interactions between oppositely charged proteins and polysaccharides are of particular interest in food science, especially with respect to functional foods. The nature of interactions between proteins and polysaccharides, whether repulsive or attractive, will lead to different behaviors as segregative or associative phase separation, respectively. The associative phase separation can be a consequence of a complexation process. Such phenomenon is initiated at themolecular scale through attractive electrostatic interactions between one polysaccharide molecule and one (or more) protein molecule. When two polymers are oppositely charged, an electrostatic complex can be formed. The electrostatically bound complexes can be either soluble or insoluble. The insoluble complexes concentrate in liquid coacervate droplets, that further coalesce and phase separate to form a separate coacervate layer. As a result, one phase of the mixture is concentrated in the two polymers and the other phase contains mainly the solvent. Proteins and polysaccharides are widely used in food products. As the two biopolymers are often used simultaneously, understanding their interactions has great importance in control of structure and texture of manufactured foods. In this study, interactions between ?-lactoglobulin (BLG) and tragacanth gum (TG) were studied in aqueous solution, at pH ranging from 3.5 to 5.5 and at different concentrations of the biopolymers. BLG and TG solutions were prepared in distilled water under gentle stirring at 20°C. The dispersions were kept at 4 °C for 18 h to allow complete hydration of biopolymers. Different biopolymers concentrations were mixed and pH value was adjusted by HCl. Absorbance of the mixtures were measured at 650 nm using spectrophotometer. The measured absorbances were plotted against pH. After mixing of biopolymers in specific ratios and adjusting pH value, the phase diagram was obtained. In this study, pH C , pH ? , and pH opt were identified and measured for different biopolymers solutions. The formation of intrapolymer complexes is believed to be initiated at pHc. By decreasing pH value of the biopolymers mixture, firstly, the biopolymers complexes were observed and mixture absorbance was increased. When pH value was dropped under certain value, phase separation was observed. The pH at which complexes or phase separation occurred depended on protein/polysaccharide ratios and by increasing BLG quantity in the mixtures, critical pH was near the isoelectric pH of BLG. Result showed that TG has an interesting phase diagram comparing other gums and phase diagrams of BLG and TG showed that the phase separation observed during experiments is an association phase separation. Rheological studies showed an increase in viscosity in first step of solutions complex for