The cyclic voltammetric responses and capacitive behavior of single-walled carbon nanotube sheets or papers are described. Broad redox responses have been observed in aqueous solutions that are probably due to the presence of oxygen-containing groups bound to the surface of the nanotubes or to the impurities produced during nanotube purification. The voltammetry and capacitance of the nanotube paper do not vary significantly when the chemical nature of the electrolyte ions is changed. In nonaqueous media, no redox responses are produced except in solutions of Li1, where an intense reduction response, possibly due to lithium insertion into the nanotube bundles, is observed. Electrochemical impedance spectroscopy in aqueous solutions reveals the typical features associated with a porous material. Electrochemical quartz crystal microbalance studies in both aqueous and nonaqueous electrolytes show that the mass of the nanotube film increases as the potential is made more negative. These mass changes are affected by, but are not directly proportional to, the cation mass. In acetonitrile solutions of Li1, a significant increase in mass associated with the suggested insertion of the cation is observed.