The electrochemical behavior of Li/LiPF6(1 M EC-DEC)/LiMnO2 cells has been investigated in previous studies. Here we report the results of extensive in situ X-ray studies which have been carried out in order to map out the evolution of the crystal structure upon subsequent charge and discharge of the cathode material. It has been confirmed that LiMnO2 converts irreversibly to a cubic structure LiMn2O4 after the initial charge (equal to delithiation) of the cathode. A second (tetragonal) phase is observed on subsequent discharge on the 3.0 V plateau. The evolution of the lattice parameters of the tetragonal phase as a function of cycle number is reported as the material undergoes this transition each cycle.