The exchange equilibria represented by the systems: (i) H-clay + MOH, (ii) H-clay + MCl and (iii) M-clay + HCl, have been studied at different electrolyte concentrations by measuring the ionic activities with the help of clay-membrane and glass electrodes. The neutralisation curves (A) with the three bases, viz., sodium, potassium and calcium hydro­xides, show a gradual rise of pH till about 90% neutralisation, when a sharp rise is observed. The metal-ion activity curves show a small initial lowering, followed by a sharp rise, on further addition of alkali. Addition of hydrochloric acid to the base-saturated clay increases both flit hydrogen-ion and the metal-ion activities very sharply. The corresponding curves obtained for the reverse reaction, by the addition of neutral salts to hydrogen clay, show a very slow rise, excepting in the potassium system, of hydrogen-ion activity, but a sharp rise in metal-ion activities.