Abstract The addition of organic cationic compounds to an aqueous suspension of a clay modifies the surface properties of the particles, and makes them wettable by organic solvents rather than by the water. The organic cations are adsorbed on the edges of the clay particles, exchanging the previous inorganic counter-ion. This is due to a strong preference of the clay for the organic cation until all the available sites will be exchanged and the clay becomes lipophilic. The stability of the resulting dispersions in oil depends strongly on the ratio between organic cation and clay. The optimum ratio is displayed near the equivalent point of ionic exchange of the clay (c.e.c.), after which a peptization occurs and the clay again becomes lipophobic and unsuitable for the dispersion of the clay in organic solvents. In this paper we describe the study of the surface changes and the rheological properties of a system composed of a water dispersion of ammonium montmorillonite, in relation to the addition of hexadecyl pyridinium chloride. The UVP (ultrasonic vibration potential) technique has been employed. The data obtained are in agreement with the rheological measurements (zero shear viscosity, pseudo-newtonian viscosity). The flocculation-deflocculation process occurring during the addition of the organic cation is discussed.