In situ transmission FTIR spectra are measured during the electrochemical insertion of lithium into phospho-olivine FePO4. The spectroelectrochemical cell consists of a composite FePO4 cathode, a lithium metal anode, and an electrolyte of 1M LiPF6 in a 1:1 mixture of ethylene carbonate and diethyl carbonate (EC–DEC). Bands belonging to the electrolyte and cathode are identified in the infrared spectra of the in situ cells. The antisymmetric PO4 3− bending vibrations (ν4) are used to monitor Li+ insertion into FePO4. Discharging produces spectral changes that are consistent with the formation of phospho-olivine LiFePO4, yet the electrolyte bands are not affected by the discharging process. The in situ infrared experiments confirm the two-phase mechanism for lithium insertion into FePO4. Moreover, the experiments demonstrate the ability to collect in situ transmission FTIR spectra of functioning electrode materials in lithium batteries. Unfortunately, lithium plating occurs on the optical window when the Li//FePO4 half-cells are charged. The use of an intercalation anode such as graphite could alleviate this problem; however, this avenue of research is not explored in this study.