The bulk resistance of ionophore-based ion-selective electrodes (ISEs) is studied by means of chronopotentiometry and electrochemical impedance using Ca ²⁺-ISEs with PVC membranes containing ETH 1001 as a model system. It is shown that the membrane bulk resistance is roughly the same within the range of CaCl ₂ concentrations from 0.3 to 10 ⁻³ M, and increases significantly when the concentration is decreased to 10 ⁻⁴ and 10 ⁻⁵ M, although the potentiometric response of the ISEs is Nernstian in all these solutions. This increase of the resistance when 10 ⁻³ M CaCl ₂ is further deleted is larger than the decrease of the resistance when CaCl ₂ is replaced with Ca(SCN) ₂ and Ca(ClO ₄) ₂. The results suggest that the non-constancy of the ISE bulk resistance is not caused by co-extraction of CaCl ₂ and therefore is not in conflict with the Nernstian behavior of the IESs potentials. The effect is tentatively ascribed to the regularities of water uptake by ISE membranes.