The influence of counterion size on short range repulsive forces at high salt concentrations was investigated with Al2O3 slurries at pH 12 coagulated with the chlorides of Li+, Na+, K+, Cs+, and TMA(+) (tetramethylammonium)(1+). Measurements of viscosity, shear modulus, and yield stress of slurries, as well as the relative density and flow stress, of saturated, consolidated bodies were performed. The results clearly show that the range of the repulsive forces correlated with the size of the unhydrated ion; namely, stronger particle networks are achieved with smaller bare counterions. Our findings are contradictory to the widely accepted hydration force model, which attributes short range repulsive forces to the desorption of fully hydrated cations as surfaces are pushed together. However, the results are consistent with recently developed statistical mechanics models describing the interaction of ions of different sizes with surfaces and their hydration layers.