Viscosity prediction of carbonaceous solid-water slurries is essential to process design and control. Existing viscosity models take into account particle-size distributions but fail to account for the surface chemistry of the solids. Surface chemistry affects inter-particle and particle-water interactions and therefore influences slurry viscosity of concentrated suspensions. On the basis of surface chemistries characterized by contact angle and zeta potential measurements, this study determines the inter-particle interaction energies. Polar interaction energy, observed to be 2-3 orders of magnitude greater than the electrostatic interaction energy and the van der Waal interaction energy, is clearly the dominant interaction energy for such a system. Both hydrophobic and hydrophilic interactions of these particles in water result in microstructures which trap water either in the form of coalescing droplets in aggregation networks or in the form of hydration layers around carbonaceous solids, leading to an increase in the effective solid volume fraction and thus increase in slurry viscosity. The increase in effective solid volume fraction is dependent on the surface chemistry of the solid and thus is specific to a carbonaceous solid. The factor by which solid volume fraction increases for a particular carbonaceous solid was determined using viscosity measurements and Krieger-Dougherty equation and was found to correlate very well with the oxygen to carbon ratio of the solid. Incorporating this factor in the Krieger-Dougherty equation resulted in accurate prediction of slurry viscosity. This modified model was successfully validated using three other concentrated carbonaceous solid-water slurries.