Conducting polymer-coated electrodes have been studied as one of the most suitable solid electrodes in organic membrane for fabrication of all-solid ion-selective electrodes or the voltammetric devices for the ion transfer at the liquid-liquid interface. In the present work, we report that the partially oxidized conducting polymer-coated electrode the 50% oxidized poly(3,4-ethylenedioxythiophene), PEDOT,-coated indium-tin oxide glass electrode, ITOE has high potential stability, reproducibility of the potential and low susceptibility to current flowing in a hydrophobic organic phases such as 1,2-dichloroethane, DCE, and 2-nitrophenyloctylether, NPOE; potential drift in open circuit potential for 200 h in DCE and NPOE were < 11 mV, reproducibility of the electrode potential in DCE and NPOE were +/- 10 mV, and the maximum redox capacity allowing potential drift < 10 mV were 0.100 mC cm(-2) in DCE and 0.113 mC cm(-2) in NPOE, respectively. These results imply that the 50% oxidized PEDOT-ITOE functions as both the reference electrode and the counter electrode in organic membrane for an amperometric ion sensor. The oxidation ratio in the PEDOT film was the primary factor influencing the potential stability. The optimum oxidation ratio was evaluated from the dependence of the absorption spectra of the conducting polymer on the applied potential, and adjustment method of the optimum oxidation ratio was proposed. The 50% oxidized PEDOT-ITOE is stable in air within 200 h, but oxidation by air cannot be ignored for a longer time period of 30 days. The proposed 50% oxidized PEDOT-ITOE is applicable for incorporation in a semi-disposable device used for 1 week. Effect of the oxidation ratio in PEDOT on the voltammetry for the ion transfer was also examined, and the 50% oxidized PEDOT-ITOE was usable in voltammetric thin layer cell of the two-electrode system based on the ion transfer at the water-organic membrane interface.