Comparison is made between the use of either a carbon powder or a polymer additive to the precursors in the synthesis of LiFePO4 from the Fe(III) compound FePO4(H2O)2 and Li2CO3. The evolution of the structural properties and phase purity with temperature and time have been monitored at all length scales by X-ray diffraction, Fourier transformed infrared spectroscopy, and magnetic susceptibility. The reactor temperature was decreased to 300 °C to investigate the early stages of the reaction. Formation of crystalline LiFePO4 begins in the range 300-400 °C only if the polymer is used as the carbonaceous additive. This LiFePO4 formation is made possible by the reduction of Fe(III) species by gases such as H2 or gaseous hydrocarbons evolved during the calcination of the polymer. Moreover, decomposition of the polymer results in a carbonaceous deposit on the surface of the LiFePO4 particles. An Li3Fe2(PO4)3 impurity found after sintering at 400 °C for 4 h was greatly reduced after sintering at 400 °C for 24 h, and phase- pure LiFePO4 was attained at 700 °C. Where the solid carbon powder was used as the reducing agent, no Fe(II) species could be detected after sintering at 400 °C. Carbothermal reduction of Fe(III) is ruled out in this temperature range.