Abstract: Homoionic samples of four montmorillonite clays, selected on the basis of the properties which control their economic uses, were prepared by treating with two normal neutral solutions of ammonium acetate, lithium, sodium, potassium, calcium, and magnesium chloride. Atterberg plastic and liquid limits, slopes of the liquid limit curves, and water sorption curves were determined. Among the results suggested, the following are particularly considered: The plastic state is reached when enough water is put into the system to fill all the pore space and to supply all the montmorillonite layers with water adequate for rigid layers plus a slight additional amount. The type of cation is not the only factor which controls the properties of montmorillonite. The structure of the lattice also appears to determine the properties of montmorillonite.