Bulk resistance and other electrochemical properties of membranes of K+-selective electrodes (ISEs) containing valinomycin are measured by means of chronopotentiometry and electrochemical impedance. It is shown that the bulk resistance of the membranes, within the Nernstian potentiometric response range, increases along decrease of KCl concentration in solution. Analogous results were reported earlier for Ca2+ and NO−3 ISEs. This non-constancy of the bulk resistance is in conflict with current views on the mechanism of ISEs response. Tentatively, this paradox is ascribed to heterogeneity of membranes due to water uptake from solution.