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Recent assessments in waste-to-energy technologies highlight hydrothermal liquefaction (HTL) as a suitable process for converting organic-rich waste with high moisture content into a useful resource. Organic waste materials, including sewage sludge, food, and agricultural waste residues, typically contain lipids, carbohydrates, proteins, and lignin with varying compositions depending on their origin. Fluid properties of reacting HTL slurries under subcritical water conditions, particularly viscosity and density, affect material flow and heat transfer in both batch and continuous HTL process systems. Real-time viscosity variations of 20 wt % water slurries of model compounds, sunflower oil, sucrose, and soy protein were determined in a stirred tank batch reactor using the Metzner−Otto method. Measured torque on the impeller shaft at a fixed impeller speed was used to determine the changes in the viscosity of the reacting slurry at different reaction temperatures. Changes in the viscosity of the sunflower oil mixture were insignificant as compared to viscosity variations with sugar-and soy protein-derived feedstock. The viscosity of the soy protein solution decreased rapidly with the temperature during hydrolysis of polypeptides into amino acids between 25 and 100 °C. Further increase in the temperature led to minimal changes in viscosity as more soluble compounds were produced above 100 °C. The viscosity of the sucrose solution changed significantly above 250 °C when carbon compounds precipitated from the solution. Viscosity variations of mixtures of model compounds were determined to predict viscosity changes in biomass.