The thermal behavior of FePO4 between 500°C and 1250°C at atmosphere pressure in air was studied by differential thermal analyses (DTA), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). The low-temperature, quartz-like trigonal phase of α-FePO4 forms on quenching from all temperatures examined. FePO4 reduces to form Fe2P2O7 when heated above 1000°C in air. The change in Fe2+/Fe3+ ratio as a function of time and temperature was calculated from weight losses from quenched samples, and the enthalpy change for the reduction reaction was determined to be 185 kJ/mole. Copyright © 2007 MS & T'07®.