Viscosity is a key property affecting reactor and process design in many industrial applications. For the conversion of biomass to ethanol to become economical on an industrial scale, biomass slurries will likely need to consist of high solids concentrations in order to reduce water usage, minimize reactor volumes, and maximize product concentration in the fermentable sugar stream. However, high solids concentrations directly impact the slurry viscosity and lead to high power consumption which is often a limiting factor at industrial scale. Optimizing the solids concentration will require balancing viscosity with these other operating requirements. Viscosity of corn stover and sawdust slurries have been characterized for a range of initial solids concentrations, and changes in solids content and viscosity are tracked as the solids are degraded by enzymatic hydrolysis. SEM pictures are used to illustrate how surface characteristics of the particles affect the slurry viscosity, and how the properties of the particle surface are affected by the enzyme which leads to changes in viscosity. Furthermore, different size particles are seen to have differing surface features which also affect viscosity.