Potentiometric ion sensors were prepared from the conjugated polymer poly(3-octylthiopene) ( POT). The influence of additional membrane components, including silver 7,8,9,10,11,12-hexabromocarborane (AgCB11H6Br6) and potassium tetrakis[3,5-bis(trifluoromethyl) phenyl] borate (KTpFPB) as lipophilic salts, and [2.2.2] p, p, p-cyclophane as silver ionophore, was studied. The membrane components were dissolved in chloroform and membranes were prepared by solution casting on glassy carbon disk electrodes. For comparison, POT-based potentiometric sensors were also prepared by galvanostatic electrosynthesis of POT from the 3-octylthiophene monomer. All the POT-based ion sensors fabricated by solution casting show Nernstian or slightly sub-Nernstian response to Ag+, even those based only on POT without any additional membrane components. The potentiometric response of electrochemically polymerized POT depends on the film thickness and the doping anion incorporated in the conducting polymer during polymerization. It is of particular importance that chemically synthesized undoped POT (without any additives) shows a sensitive and selective potentiometric response to Ag+ ions although UV-vis results show that POT remains in its undoped form, i.e., POT is not oxidized by Ag+. This indicates that undoped POT can exhibit good sensitivity and selectivity to Ag+ also in the absence of metallic silver in the polymer film. In this case, the potentiometric response is related to interactions between Ag+ and the conjugated polymer backbone.