Reversible insertion of Li into purified single wall carbon nanotubes was achieved electrochemically. Galvanostatic charge-discharge and cyclic voltammetry indicated that there is no well-defined redox potential for Li insertion or removal in the nanotube lattice. The Li reversible capacity was found to be 460 mA.h/g, significantly higher than the theoretical value for graphite.In-situX-ray diffraction revealed an irreversible loss of the 2-D triangular lattice upon doping.In-situresistivity measurements presented a 20-fold decrease in resistance upon doping, reversible upon undoping.