Four commercial carbon materials, carbon nanotube, active carbon, acetylene black, and graphite, were treated by concentrated nitric acid. The surface properties and the electrochemical capacitance of the treated and the untreated carbon samples were studied by using scanning electron spectroscopy, BET surface analysis, constant current charge/discharge test, cyclic voltammetry, and alternative current impedance. It is found that the untreated samples have different specific capacitance and specific surface area, which are in the order from large to small: carbon nanotube, active carbon, acetylene black, and graphite. After treated with nitric acid, the specific surface area of these commercial carbon materials increases to different extents, and the specific capacitance of carbon nanotube, acetylene black and graphite increases proportionally to their specific surface area but the specific capacitance of active carbon decreases. The effect of acid treatment on the capacitance of the commercial carbon samples is related to their porosity structure and surface functional groups. (C) 2008 Elsevier B.V. All rights reserved.