The carbon aerogel (CAG)-multiwalled carbon nanotubes (MWNTs) nanocomposite binderless electrodes and their electrochemical performances in 3 M H2SO4 and 5 M KOH were investigated. Additionally investigated was their long life. We measured the highest capacitances with an optimum CAG-MWNT composite containing 3 wt % of MWNT displaying a specific capacitance of 232 F/g in H2SO4 vs 218 F/g in KOH. The highest capacitance was obtained with the CAG-3 wt % MWNT because this sample displayed the highest content of oxygen, and the highest ultramicropore volume that can accommodate electrolyte ions (OH- in KOH and SO42- in H2SO4). The capacitance of these materials when subjected to several thousands of galvanostatic charge-discharge experiments revealed that the samples showed a fairly stable behavior in KOH electrolyte, whereas in H2SO4 electrolyte only the composite containing the highest amount of MWNTs (i.e., 12 wt %) was stable. The other samples displayed a dramatic enhancement in the capacitance during the first few thousands of cycles and then dropped to very low values. This observed phenomenon was ascribed to the swelling and attack of the internal carbon atoms in a concentrated acidic medium. (c) 2007 The Electrochemical Society.