The thermodynamic activities of ion pairs were measured electrometrically in a series of aqueous mixtures of monoionic bentonite and the corresponding chlorides. Ion exchange membranes and Ag-AgCl electrodes were used. The lyotropic order for the various cation species varied somewhat for two different preparations. The apparent relative escaping tendencies of the various cations did not correspond to that usually assumed on the basis of related phenomena: ion exchange selectivity, electromigration velocities, flocculation values or rates, etc.