Effective and efficient breakdown of lignocellulosic biomass remains a primary barrier for its use as a feedstock for renewable transportation fuels. A more detailed understanding of the material properties of biomass slurries during conversion is needed to design cost-effective conversion processes. A series of enzymatic saccharification experiments were performed with dilute acid pretreated corn stover at initial insoluble solids loadings of 20% by mass, during which the concentration of particulate solids and the rheological property yield stress (tau(y)) of the slurries were measured. The saccharified stover liquefies to the point of being pourable (tau(y) <or= 10 Pa) at a total biomass conversion of about 40%, after roughly 2 days of saccharification for a moderate loading of enzyme. Mass balance and semi-empirical relationships are developed to connect the progress of enzymatic hydrolysis with particle concentration and yield stress. The experimental data show good agreement with the proposed relationships. The predictive models developed here are based on established physical principles and should be applicable to the saccharification of other biomass systems. The concepts presented, especially the ability to predict yield stress from extent of conversion, will be helpful in the design and optimization of enzymatic hydrolysis processes that operate at high-solids loadings.