Hydration of oat bran including fermentation by rye sourdough was studied. Three types of oat bran suspensions were prepared (a control, one with whole meal rye flour added, and one with rye starter added). The suspensions were incubated for 1, 2, 3 and 4 hr. β-Glucan content and solubilities of protein and β-glucan were analyzed. Viscosity of the supernatants of oat bran suspensions was determined. Neither the rye sourdough nor the rye flour alone had a significant effect on the total β-glucan content of oat bran suspensions. However, the addition of rye, either as whole meal rye flour or as sourdough starter, markedly increased the solubility of β-glucan and proteins and simultaneously decreased the viscosity of the water-soluble fraction of oat bran suspension. This suggests that a hydrolysis of β-glucan had occurred that could change the rheological properties of oat bran in baking and the physiological potential of oat bran in nutrition.