Grinding tests were conducted under various conditions to characterize the breakage characteristics of coal-water mixtures. Five bituminous coals were ground in a conventional tumbling ball mill for different times at solids concentrations of 40-70% by weight, with and without the use of a dispersant. For a given coal, the grinding rate decreased as the solids concentration increased, whereas grinding at a higher solids concentration produced a higher proportion of fines, giving a flatter distribution. However, even at the highest slurry density, the product size distribution was steeper than the broad size distribution recommended for maximum solids loading and minimum viscosity. Additional grinding tests were performed in a stirred ball mill, aiming at a two-stage grinding method for producing coal-water mixtures. On the basis of the breakage parameters obtained from the laboratory tests, simulations were performed to analyze an industrial scale two-stage grinding circuit.