Fundamental complexities related to the studies on interfacial charge transfer at ionophore-based ion-selective electrode (ISE) membranes have been briefly discussed. The presented data obtained from ac-impedance studies on ISEs selective to K⁺, Na⁺, Li⁺, and Ca²⁺ ions have shown fast interfacial transfer of primary ions, and also of interfering ions, supporting the concept of fast establishment of interfacial electrochemical equilibrium towards all competing ions in the solution.