We have used for the first time a conjugated redox polymer with hydroquinone (HQ) pendant groups covalently attached to the poly(3,4-ethylenedioxythiophene) (PEDOT) backbone as the solid contact (SC) in plasticized poly (vinyl chloride) (PVC) based K+-selective electrodes (K-SCISE). Redox couples are one of the simplest ways to precisely adjust the standard potential (E◦) of the SCISEs, but usually the initially high E◦ reproducibility is lost quite quickly due to leaching out of non-covalently bound redox molecules from the SCISE. In PEDOT-HQ, the covalently attached HQ groups prevent the leaching and simultaneously allow additional charge storage in PEDOT-HQ that is ca. 25–30 times higher than for unsubstituted PEDOT. Before the ion-selective membrane (ISM) deposition, we controlled the potential of the SC with high reproducibility (±0.4 mV, n = 5) by prepolarization in a mixture of acetonitrile containing potassium tetrakis(pentafluorophenyl)borate and perchloric acid as proton source. Pre-polarization of the SC close to the formal potential where the redox buffer capacity is highest gave the best potential reproducibility. However, after the ISM deposition, the K-SCISEs showed in the best case an E◦ reproducibility of ±2.8 mV (n = 5). Chronopotentiometric measurements reveal that only a minor fraction of the very high redox capacitance of PEDOT-HQ can be utilized for the ion-to-electron transduction beneath the ISM. The influence of this shortcoming on the E◦ reproducibility of the SCISEs has been underestimated for most SC materials. Modification of the commonly used PVC-ISM formulations to allow faster ion transfer at the SC/ISM interface could be one way of overcoming the disadvantage.