Abstract: Kaolinite, to which iron oxides had been added by heating with FeCl3 solution, was studied with regard to its cation exchange capacity. It was observed that net cations held (cations less anions) varied directly with pH. Cations and anions are removed in equivalent amounts when the clays are washed with water. Removal of exchangeable cations proceeds as the last anions are removed. As a consequence, the negative sites may become saturated with non-exchangeable hydroxy-aluminum ions. To obtain the true CEC of a kaolinite-iron oxide system, it is necessary to estimate the amount of nonexchangeable hydroxy-aluminum by conductometric titration.