A Li x MnPO4 (x =1.0–0.15) composite cathode was investigated by Fourier-transform infrared spectroscopy at different states of charge. Significant spectral changes of the PO4 3− vibrations, which are correlated with the Jahn–Teller distortion of Mn3+ in MnPO4 and the 3rd ionization potential of Mn, were observed upon electrochemical delithiation of LiMnPO4. The presence of two sets of peaks observed in the series of delithiated Li x MnPO4 spectra is consistent with a two-phase process for delithiation. These results provide insight into the structural changes that occur during lithium extraction and insertion in LiMnPO4.