Phase separation is a common phenomenon in biopolymer mixtures. In food systems, the biopolymers are represented by proteins and polysaccharides. Those two biopolymers play a major role in structuring foods and determining the physicochemical properties of food products. Depending on the interactions between two biopolymers, segregative or associative phase separation can take place upon mixing. Segregative phase separation occurs when two different biopolymers are repelling each other, whereas associative phase separation occurs when two oppositely charged biopolymers are mixed. Predicting phase separation is therefore of importance to design and control (novel) food processes and products, as the microstructure of a composite food product depends critically on the phase behaviour of the biopolymers which it contains. This PhD thesis provides a novel approach on the basis of a virial model to describe segregative phase separation in biopolymer mixtures, where the second virial coefficients can be extracted from the phase diagram. New analytical expressions for determining the characteristics of the phase diagram are also given in this thesis. For associative phase separation, it was found that membrane osmometry could be a useful technique to quantify attractive interactions between proteins undergoing a complexation.