The potentiometric response and the electrochemical impedance spectra of thin PVC based membranes selective to Ca²⁺ and to Na⁺ were studied before and after first contact with respective primary ions. The membranes included potassium tetrakis(p-Cl-phenyl) borate, thus being initially in potassium form. Nearly Nernstian potentiometric responses were obtained in interfering electrolytes before the contact with primary ions, while after the first contact the slope of potentiometric curves decreased drastically. However, the impedance spectra were similar regardless of prior conditioning of the membranes, and no evidence for a hindrance towards charge transfer processes was observed. The results suggest that the membrane/solution interface reactions are reversible both for the primary and the interfering ions, and the contamination of the solutions with primary ions is the only reason for the lack of Nernstian response in interfering electrolytes.