The zeta potential of different clays such as kaolinite, montomorillonite, and fuller's earth at 100, 250, 500, and 1 000 mg/l in 0.145, 0.58, 1.45, and 5.8 times 10‐4Mof aluminium sulphate at various pHs has been studied and compared with results for a surface water. The clays were selected on the basis of cation‐exchange capacity. The results are discussed with respect to the isoelectric point as an indicant for achieving optimum destabilization with an economically efficient coagulant dose.