Enzymatic hydrolysis of cellulose for conversion to chemicals or fuels presents engineering challenges due to the large changes in suspension viscosity and yield stress that occur. A flow reactor with an in-line rheometer was used to investigate the role of changes in fiber structure on rheology. The evolution of the suspension yield stress was compared to amount of soluble sugars released and changes in fiber length and width. A model was constructed that links the yield stress, conversion, and fiber shape. These results provide insights into the relationship between fiber structure and transport properties during the early stages of hydrolysis of cellulosic biomass. (c) 2014 American Institute of Chemical Engineers AIChE J, 60: 1582-1590, 2014