The energy storage potentials of many different carbon nanotube (CNT) materials are investigated electrochemically. The electrochemical response of all the investigated materials is rather featureless during charging and discharging and does not show any phase transitions or clear redox responses. The maximum discharge capacity, similar to 130 mAh/g, is measured for an as-produced single-walled carbon nanotube material. Measurements indicate that the electrochemical activity of the matrix material, used to manufacture composite electrodes, should also not be ignored. Highly pure CNTs yield a substantially lower response, indicating that the CNTs itself can only account for a small part hereof. The measured response of CNT materials is related to a number of processes, including the irreversible oxidation of carbonaceous material, the reversible oxidation/reduction of residual metal catalyst or carbonaceous impurities, and an electrostatic charging component. Steady-state impedance measurements, cross-correlated with cyclic voltammetry, show that (dis)charging of the electrical double layer can be directly linked to this electrostatic charging component. Characterization of highly pure CNT materials shows that more than 90% of the total charge was stored in this way. Only about 25% of the total amount of charge could be explained in this way for the as-produced materials. Based on the overall results it is highly unlikely that a significant amount of hydrogen can be stored in CNT material and that the exact amount of charge that can be reversibly stored in CNT material heavily depends on its morphology and the level of purity. (c) 2006 The Electrochemical Society.