The cyclic voltammetry of single-wall carbon nanotube films in aqueous solutions show the presence of broad redox responses. In non-aqueous media there is absence of redox processes when the electrolyte cation is tetrabutylammonium. However, in a Li+-containing organic electrolyte, reduction associated with lithium insertion occurs over a narrow potential range. Electrochemical quartz crystal microbalance studies in aqueous electrolytes show that the electrode mass increases with increasing negative potentials. These mass changes as a function of applied potential are not directly proportional to cation mass and are associated with double-layer charging. For electrolytes with cations larger than Li+, the change in electrode mass is smaller than the mass of the anions or cations involved. The mass changes of nanotube films in acetonitrile solutions are similar to those observed for aqueous electrolytes when the cation is tetrabutylammonium. However, solutions of Li+ cause a substantial increase in nanotube film mass associated with cation insertion, which is not readily reversed reflecting slow Li+ expulsion. (C) 2000 Elsevier Science Ltd. All rights reserved.