ζ-potentials of insulating plates, e.g., quartz and sapphire plates, have been measured by means of the plane interface technique. This technique involves determining the electro-osmotic flow velocity-depth profile of reference (polystyrene latex) particles inside a cell. The electro-osmotic velocity at the lower boundary of the cell, referring to the sample surface-solution interface, permits to calculate the ζ-potential of the sample. However, this method is inapplicable to electrically conductive plates such as metal, because its conductivity results in electrolysis of water and disturbs to establish the electro-osmotic velocity profile. It is proved to be efficient for measuring the ζ-potential of metallic plates by inserting an insulator between the cell and the conductive plate. The size of the spacer is 4 mm inside width and 45 μm thick. The precision and accuracy of this method were proved to be ± 10 mV. This method is applicable to evaluate the adsorption between the aqueous ions and the aluminum alloy plates.