Shear flow in α-A12O3 suspensions having a volume fraction of solids (φ) in the range between 0.17 and 0.50 was investigated between pH 4 and 12. It is Newtonian if the magnitude of the zeta potential exceeds a critical value which depends on φ;its value is 39 mV if φ= 0.40 and 74 mV if φ= 0.50. If this potential is less than the critical value, shear flow is pseudoplastic; its yield value markedly changes (e.g., 0 to >100 Pa) in a slightly φ-dependent, narrow pH range (<0.5 units). If a second oxide, t-ZrO2 , is present, its pH-dependent colloidal behavior governs the overall rheology, though its concentration may be only 11 % that of α-A12O3. Scanning electron microscopy of composite pieces indicates that a detrimental, rheologically detectable interaction between α-Al2O3 and t-ZrO2 can be avoided and the distribution of t-ZrO2 can be optimized during pressure casting by control of pH. Best conditions correspond to either Newtonian flow or pseudoplastic flow with a very low yield value.