Electrochemical properties of poly(3,4-ethylenedioxythiophene) doped with hexacyanoferrate(II,III) ions (PEDOT(HCF)) were studied in the presence of Cu2+ ions. Voltammetric and EDAX studies revealed retention of hexacyanoferrate anions in the polymer film and accumulation of Cu(II) cations, as well as formation of solid copper hexacyanoferrate near the polymer surface. Accumulation of Cu2+ ions was found to be advantageous from the point of view of PEDOT(HCF) applications as a solid contact (ion-to-electron transducer) in all-solid-state Cu2+-selective electrodes with solvent polymeric polyvinyl chloride (PVC) based membrane, containing Cu2+-selective ionophore. Binding of Cu2+ ions in the conducting polymer layer results in analyte ions flux into the transducer phase. Thus, pronounced enhancement of selectivity of the all-solid-state Cu2+-selective electrode or lower detection limit of the potentiometric response range was achieved, reaching under optimised conditions 10−7 M CuSO4.