Investigations were carried out on methods of measuring the cation exchange capacity (CEC) of clay suspensions. A methylene blue method was developed and found useful because of the simple operations involved and the yielding of reproducible results. Possible applications of CEC data were explored. The control of pH in coagulation of kaolinitic and bentonitic suspensions produced a significant effect on turbidity removal. The optimum pH of coagulation was shown to be dependent on the type and concentration of clay and the alkalinity of the water.