Li4/3Ti5/3O4 is a good anode material for rechargeable lithium batteries. This material exhibits characteristic properties, including very flat discharge and charge curves and an infinitesimal structural change during discharge and charge. In this study the structural behaviour was confirmed by the Rietveld analysis of X-ray diffraction patterns: in situ UV-visible spectroscopy of Li4/3Ti5/3O4 during discharge was also performed to observe the electronic structure change induced by lithium insertion. The Rietveld analysis clearly showed that no structural change could be detected during the lithium insertion and extraction processes. The UV-vis. spectroscopy revealed that the insertion of lithium into Li4/3Ti5/3O4 results in the formation of a new phase with the same lattice constant. These results indicate that the insertion and extraction of lithium into and from Li4/3Ti5/3O4 proceed via two-phase reactions, while the lattice parameter is the same as that of Li4/3Ti5/3O4 with lithium insertion.