The present paper shows that the performance of an inexpensive activated carbon used in electrochemical capacitors can be significantly enhanced by a simple treatment with KOH at 850 degrees C. The changes in the specific surface area, as well as in the surface chemistry, lead to high capacitance values. which provide a noticeable energy density. The KOH-treatment of a commercial activated carbon leads to highly pure carbons with effective surface areas in the range of 1300-1500 m(2) g(-1) and gravimetric capacitances as high as three times that of the raw carbon. For re-activated carbons. one obtains at low current density (50 mA g(-1)) values of 200 F g(-1) in aqueous electrolytes (1 M H2SO4 and 6M KOH) and around 150 F g(-1) in 1 M (C-2 H-5)(4)NBF4 in acetonitrile. Furthermore, the resulting carbons present an enhanced and stable performance for high charge/discharge load in organic and aqueous media. This work confirms the possibilities offered by immersion calorimetry on its own for the prediction of the specific capacitance of carbons in (C2H5)(4)NBF4/acetonitrile. On the other hand, it also shows the limitations of this technique to assess, with a good accuracy, the suitability of a carbon to be used as capacitor electrodes operating in aqueous electrolytes (H2SO4 and KOH). (C) 2007 Elsevier Ltd. All rights reserved.