The effect of ion size on the force necessary to remove counterions from the surface when particles are forced into contact (primary minimum) was investigated. Alumina slurries dispersed at pH 12 and then coagulated with 0.5 M Li+, Cs+, and tetraethylammonium (TEA(+)) chlorides were consolidated by pressure filtration at different applied pressures. Uniaxial compression tests were performed to determine the plastic-to-brittle transition pressure for bodies formulated with each of the different counterions. Brittle bodies are produced when particles are pushed together to form a strong touching network. The force required to push particles together is related to the slope of the repulsive potential barrier, with steeper slopes requiring greater force. The consolidation pressure necessary for brittle behavior was greater for slurries with small counterions such as Li+ as compared with Cs+ and TEA(+). The results presented here show that the resistance to pushing particles into contact is related to the size of the bare counterion in weakly attractive (salt-coagulated) slurries. That is, the slope of the potential barrier produced when the slurry is formulated with smaller ions is steeper. This result is consistent with the idea that smaller counterions are more strongly bound to the surface, as well as the fact that the repulsion due to the finite size of the ion occurs at smaller interparticle separation distances for the Smaller counterions.