Electrochemical characteristics of electrodes for supercapacitors built from RuO2/multiwalled carbon nanotube (CNT) nanocomposites have been investigated. Capacitances have been estimated by cyclic voltammetry at different scan rates from 5-50 mV/s. Electrostatic charge storage as well as pseudofaradaic reactions of RuO2 nanoparticles have been affected by the surface functionality of CNTs due to the increased hydrophilicity. Such hydrophilicity enables easy access of the solvated ions to the electrode/electrolyte interface, which increases faradaic reaction site number of RuO2 nanoparticles. The specific capacitance of RuO2/pristine CNT nanocomposites based on the combined mass was about 70 F/g (RuO2 : 13 wt % loading), and the specific capacitance based on the mass of RuO2 was 500 F/g. However, the specific capacitance of RuO2/hydrophilic CNT nanocomposites based on the combined mass was about 120 F/g (RuO2 : 13 wt % loading), and the specific capacitance based on the mass of RuO2 was about 900 F/g. (C) 2003 The Electrochemical Society.