Reversible insertion of lithium into purified single wall carbon nanotubes was achieved electrochemically. Nanotubes exhibited reversible capacities on the order of 460 mAh/g, corresponding to a stoichiometry of Li1.23C6. The material also presented very high irreversible capacities (1200 mAh/g) which we ascribe to the large specific surface area (350 m(2)/g). Galvanostatic charge-discharge and cyclic voltammetry indicated that there is no well-defined redox potential for lithium insertion or removal in the nanotube lattice, ruling our the hypothesis of a staging mechanism via well-defined interstitial sites. In situ X-ray diffraction revealed an irreversible loss of crystallinity, suggesting that duping disrupts the intertube binding, analogous to exfoliation in layer hosts. In situ resistance measurements showed a 20-fold decrease upon doping, consistent with charge transfer between lithium and carbon. Electrochemical impedance spectra were interpreted in terms of a Randles-type equivalent circuit. The data showed a continuous decrease in charge transfer resistance upon doping, consistent with the decrease in electronic resistivity of the electrode. It also showed that the high lithium capacities are not due to double layer capacitance effects, but to an actual ion insertion/extraction process in the bulk material.