The influence of counterion size on short-range repulsive forces at high salt concentrations was investigated with silica slurries at various pHs. Ions from the lyotropic series of monovalent electrolytes (LiCl, NaCl, KCl, and CsCl) were used to coagulate dispersed slurries. Measurements of viscosity and sedimentation rate were performed. The results at high salt concentrations and volume fractions of silica clearly show that the extent of the short-range repulsive forces correlates with the size of the unhydrated ion. This trend is opposite to the ion-adsorption sequence on silica at low volume fractions of solids or lower ionic strength. Our results are also contrary to the widely accepted hydration force model but concur with the recently developed reference-hypernetted chain statistical mechanics models describing the interaction of ions with solvated surfaces.