Two brown coals of different surface charge densities and ionic strengths were evaluated for their rheological properties. Rheological behavior ranging from low viscosity Newtonian to high viscosity pseudoplastic yield, depending on the surface chemistry, was observed. By appropriately varying the ionic strength or surface charge density it was possible to convert the rheological behavior of one brown coal to that of the other. Yield stress behavior occurs when the surface charge density is low or the ionic strength is high. Conversely, the suspension is Newtonian at high surface charge density or low ionic strength. A minimum viscosity occurs when the suspension is close to the point of transition from attractive to repulsive particle interaction (i.e., just dispersed). The knowledge gained from this fundamental investigation was subsequently exploited in the development of a coal-water suspension fuel.