Fluorine-substituted LixMn(2)O(4) was studied by synchrotron X-ray diffraction using Bellcore-type plastic batteries placed directly in the synchrotron beam. The results show that the ＇4 V plateau＇ actually contains three regions: (1) a single-phase range for 0.59 less than or equal to x less than or equal to 1, (2) a two-phase range for 0.23 less than or equal to x less than or equal to 0.59, (3) a narrow single-phase range for x less than or equal to 0.23. These three domains are fully reversible, but the plastic battery undergoes extraneous oxidation reactions for voltages greater than or equal to 4.4 V. The structural features of this compound are qualitatively similar to those of stoichiometric LixMn2O4. In particular, no suppression of the two-phase range such as that observed an non-stoichiometric or Ni-substituted LiMn2O4 was found.