Potassium-selective membranes known as sensing modules for potentiometric ion-selective electrodes are studied in ac-impedance mode. It is shown that the bulk resistance of thin (20-250 μm) membranes depends on the composition of aqueous bathing solution. The results are explained in terms of surface conductivity in space-charge region, in the vicinity of water microdroplets in membranes. A planar potassium-selective sensor with a conductometric signal is proposed.