Carbon nanocoils (CNCs) are coiled carbon nanofibers of ca. 10 nm in fiber diameter, and 10-20 nm in internal coil diameter. They are prepared through several oxidation treatments, which lead to a graduate content in surface oxygen groups evolving as either CO or CO2 in temperature-programmed desorption. The CNCs show a moderate surface area (40-240 m(2) g(-1)) and a relatively high electrical conductivity (0.1-0.2 S cm(-1)). Some CNCs are activated to develop surface area. The electrochemical behaviour of CNCs as supercapacitor electrode is studied in the electrolytes H2SO4 and Et4NBF4. From the parameters C-1 (overall specific capacitance measured at 1 mA cm(-2)), SBET (BET specific surface area), and CO and CO2 content, the plots of C-1/S-BET vs. CO/S-BET and vs. CO2/S-BET are analyzed. In the acidic electrolyte, the oxygen groups evolving as CO2 contribute significantly to enhance C-1 in addition to the groups evolving as CO. C-1/S-BET shows a linear dependence, with high slope, on CO2/S-BET, and an exponential dependence on CO/S-BET. In the organic electrolyte, the two kinds of oxygen groups contribute less to C-1. The specific pseudocapacitance and double layer capacitance are determined and discussed in relation to the CO and CO2 content and the S-BET value. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.083204jes] All rights reserved.