The potentiometric response of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonate) (PSS) deposited on glassy carbon (GC), i.e. GC/PEDOT(PSS), was studied before and after conditioning in silver nitrate solution. The deposition of silver in the PEDOT(PSS) film was characterized by SEM/EDAX and cyclic voltammetry. The potentiometric response of the Ag-conditioned GC/PEDOT(PSS) electrode was dependent on the nature of the anion present in the calibration solutions. In cases where anions do not form sparingly soluble silver salts, a cationic response was observed for GC/PEDOT(PSS), as expected. However, an anionic response was observed in solutions containing anions (X−) that can form sparingly soluble salts with silver ions (AgX). Furthermore, the measured potentials in X− solutions followed the solubility product values of AgX. The agreement between the experimental and thermodynamic data was excellent. This validates the hypothesis of equilibrium existing between Ag+ ions in the polymer film (due to conditioning) and Ag+ ions in the solution. The Ag-conditioned GC/PEDOT(PSS) electrode covered with a potassium ion-selective membrane (GC/PEDOT(PSS)/K+-ISM) showed a cationic response in KCl solutions with close to Nernstian slope value.