The relationships among colloidal stability, suspension rheology, and solids loading are elucidated for zirconia-alumina mixtures containing 20 vol% ZrO2, based on solids. The lower colloidal stability of ZrO2 limits the rheological properties of this system. If the zeta potential of ZrO2 is less than 49 mV, high degrees of pseudoplasticity, high yield stress, high viscosity, and long relaxation times characterize the binary suspensions. These effects occur, despite the fact that ZrO2 is the minor ceramic constituent. If the zeta potential of ZrO2 is maintained above 49 mV, suspensions with high solids loading (55 vol%) can be prepared which behave as Newtonian fluids over the 0-to-200 s−1 shear rate range and as an elastic solid at higher rates. As the solids loading of a highly stable binary suspension is increased, the rheological properties change. They evolve from those of a near Newtonian-like fluid with nearly independent particles (40 vol%) to those of a pseudoplastic fluid with a weakly interacting particle network (50 vol%) to those of an elastic-like solid composed of crowded, strongly repulsive particles (55 vol%). Low-shear conditions for suspension-transport and mold-filling operations that ensure a homogeneous arrangement of ZrO2 and A12O3 particles in a gelcast ceramic seem promising.