The shear yield stress of alumina suspensions was measured by the vane tool technique, as a function of high concentration of various counterions. Slurries at pHs below the isoelectric point were coagulated with the sodium salts of IO3-, BrO3-, Cl-, NO3-, and ClO4-. Structure maker ions (IO3- and BrO3-) produced the strongest particle networks, while structure breaker ions (ClO4-, NO3-, and Cl-) produced weaker networks. Electrokinetic results (zeta potentials) suggest the structure makers adsorb inside the shear plane in greater quantities and are thus more efficient in collapsing the double layer. The high yield stresses in the case of IO3- cannot be fully explained by the concept of truncating the van der Waals attraction at a distance related to the size of the ion. When the possibility of either an additional attraction or a closer distance of approach is considered, the results can be adequately explained. Similar results were observed for zirconia.