Fourier transform infrared spectroscopy (FTIR) has been applied to the study of oxide insertion compounds used in rechargeable lithium batteries. The mechanisms responsible for capacity fading during normal cycling of LiMn2O4 cells in both the 3 and 4 V regions were determined by examination of spectra obtained from electrodes following 25 cycles at charge and discharge rates of C/6. In the 3 V region, electroactive material becomes electronically disconnected from the rest of the electrode due to fracture of the oxide particles and/or expansion and contraction of the active material during the cubic-to-tetragonal phase transformation. In the upper voltage region, the active electrode material is gradually converted to a lower voltage defect spinel phase via dissolution of manganese in the electrolyte.