The isothermal (5 °C) equilibrium phase behaviour of high molecular weight oat β-glucan/whey protein isolate binary mixtures was investigated. The mixtures were studied at different pH values (3.0 and 7.0) in the presence or absence of sucrose (10% w/v) by means of phase analysis, electrophoresis, viscosity measurements, and fluorescent microscopy. The mixtures are incompatible in a wide range of concentrations as revealed by phase diagram construction. Decrease of pH from 7.0 to 3.0 and sucrose addition enhances miscibility, and the latter effect is more pronounced at pH 3.0. Viscosity measurements of the mixed systems revealed that flow is pseudoplastic at all concentrations studied and the behaviour changes from protein- to polysaccharide-controlled as the biopolymer concentration varies in the mixtures. This is also reflected in the topology of the mixed systems where a transition in the continuity of the microstructure is observed from protein- to bi- to polysaccharide-continuous.