Apparent viscosities of Wiley milled white birch at various particulate sizes, suspension concentrations, and solvent and acid concentrations were measured as a function of temperature. Apparent viscosities decreased reversibly with increasing temperature below 150 °C in water in the absence of acid and/or solvent. At temperatures above 150 °C, or with added solvent or acid, the variation of the apparent viscosities with temperature became irreversible. The steady-state shear dependent behavior is well described by a Bingham model where the yield stress is a strong function of temperature and follows an Arrhenius-type behavior. The suspensions exhibited a negative plastic viscosity at low temperatures, but as the temperature increased, the plastic viscosity became less negative. This temperature dependent rheological behavior is qualitatively similar to that observed for concentrated fibrous suspensions of corn stover and synthetic fibers, indicating these changes are not unique to only birch suspensions and are associated with physical changes in the suspension properties with temperature. The irreversible viscosity changes with increase in temperature above 150 °C in water were anticipated due to partial hydrolysis of biomass in water. This was confirmed when biomass suspensions were sheared in a polar aprotic solvent (𝛾-valerolactone) and/or with the addition of sulfuric acid. Finally, the extent of irreversible change in the viscosities was correlated with the change in insoluble solids concentration for the various experiments under different conditions.