A novel method for voltammetric measurement in supercritical CO2 is described. In this method a water-in-supercritical CO2 microemulsion is utilized to raise theconductivity of the fluid phase. Well-defined voltammograms for the redox reactions of ferrocene and tetramethyl-p-phenylenediamine were obtained in supercriticalCO2 in the presence of the microemulsion. The electrolysis product is stabilized in the water core of the microemulsion. This implies a wide range of electrochemicalsynthesis involving ionic or radical species in supercriticalCO2 may be possible utilizing the microemulsion. Theelectrochemical results obtained from the water-in-supercritical CO2 microemulsion system are different from thatobtained from a supercritical CO2−ethanol mixture system.