In-situ X-ray diffraction studies have been performed on a Li4/3Ti5/3O4 electrode upon cycling in a Li cell, by using a very high energy (87.5 keV) synchrotron beam. The real time structural changes of its crystalline lattice were observed over two complete cycles of the cell. The high-resolution measurements allowed us to precisely monitor the extremely small breathing movement of the structure and to plot the curve of the lattice parameter as a function of the lithiation degree. The investigation revealed an unexpected behavior in the structural evolution upon cycling, which was attributed to the reversible passage from a monophasic to a biphasic domain upon insertion. Furthermore, the structural evolution turned out to be slightly different in the first and in the second cycle. This suggests that irreversible rearrangements, like the ones observed for every other insertion compound, occur also in this case, although on an extremely smaller scale.