Abstract: Adsorption isotherms of 1-n-alkyl pyridinium bromides on Na-montmorillonite, and in addition of cetyl pyridinium bromide on Ca-montmorillonite, have been determined. For up to eight carbon atoms in the alkyl chain adsorption has a limit close to the exchange capacity of the clay. With larger ions adsorption occurs beyond this and is accompanied by adsorption of the bromide ion. Adsorption of the cetyl pyridinium ion beyond the exchange capacity is greater on Na- than on Ca-montmorillonite. Replacement of the cation initially present is incomplete. X-ray diffraction analysis shows that the adsorbed ions normally lie flat on the clay surface, but cetyl pyridinium ions may stand up in a plane at right angles to the surface. Interlamellar separations somewhat less than the minimum molecular thicknesses, found for the methyl and ethyl pyridinium ions, are attributed to a combination of (1) “keying” of the alkyl groups into the clay surface, and (2) the effect of the attractive forces between the clay and the pyridinium ring. It is concluded that adsorption is due to ionic and dispersion forces between the pyridinium ions and the clay. For the cetyl compound the dispersion forces are larger than ionic forces. The exchangeable cation initially present influences the adsorption by its effect on the exchange reaction and probably also by its influence on the domain structure of the clay and hence the accessibility of external surfaces of the crystallites.