High-temperature oxide melt solution calorimetry was performed on the battery material LiFePO4 and the orthorhombic and trigonal polymorphs of its delithiated form, FePO4. The enthalpies of formation from oxides and from elements at 25 degrees C were determined. The phase transition of FePO4 from orthorhombic symmetry to trigonal symmetry was investigated using differential scanning calorimetry. The enthalpies of formation from oxides at 25 degrees C for LiFePO4, o-FePO4, and t-FePO4 are -151.52 +/- 1.68, -113.68 +/- 1.26, and -102.01 +/- 1.26 kJ/mol, respectively. The enthalpy of transition from o-FePO4 to t-FePO4 is 11.67 +/- 1.56 kJ/mol. Thus the orthorhombic form of FePO4 is energetically more stable than the trigonal phase and is therefore stable at low temperatures. The equilibrium temperature of the o-t transition is probably sufficiently above room temperature, so that under the operating conditions of a battery, it is themodynamically impossible that o-FePO4 will decompose to t-FePO4. The t-o transition on cooling is kinetically hindered. (c) 2005 The Electrochemical Society.