The kinetics of phase separation and microstructure of oat β-glucan/whey protein binary mixtures varying in concentration (4-16% w/v protein, 0.3-1.2% w/v β-glucan) and β-glucan molecular weight (1.3 × 10 6, 640 × 10 3, 180 × 10 3, and 120 × 10 3 g/mol) was investigated by turbidimetry and fluorescent microscopy. The phase separation of the mixed systems was followed at pH 7.0 and at room temperature under quiescent conditions. Application of first principles revealed that phase separation of the systems follows first-order kinetics. Acceleration of the phase-separation process was observed with increase of β-glucan concentration for the three lowest-MW samples but the highest molecular weight (1.3 × 10 6 g/mol) exhibited the opposite trend. Changes in the polysaccharide molecular weight resulted in considerable differences in β-glucan aggregate morphology in the mixed systems. The change in the continuity of the mixed system from polysaccharide-, to bi-, to protein-continuous was confirmed for a wide range of mixed systems differing in biopolymer concentration, and β-glucan molecular weight.