The correlation between capacitance and the structure of carbon nanotubes (CNTs) was revealed. The electrochemical behaviors of single-and double-walled CNTs were characteristic due to the electrochemical doping mechanism of semiconducting nanotubes, which are amphoteric and can be doped by both donors and acceptors. The capacitance and the sheet conductance of CNTs, as capacitor electrodes were dependent on their porous and electronic structure. High specific capacitance was obtained for uncapped single- walled CNTs having inner micropores with a diameter of < 1 nm. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3059010] All rights reserved.