The standard molar enthalpy of formation Delta rH(m) degrees of pentamolybdenum trisilicide Mo5Si3 has been determined by fluorine combustion calorimetry to be -(314.3 +/- 8.3) kJ . mol(-1) at T = 298.15 K and p degrees = 101.325 kPa. Conventional thermodynamic properties of Mo5Si3 are tabulated to T = 2100 K. Thermodynamic quantities to the same temperature maximum are also given for the high-temperature oxidation reactions : 2MoSi(2)(s) + 7O(2)(g) = 2MoO(3)(s) + 4SiO(2)(s), and 5MoSi(2)(s) + 7O(2)(g) = Mo5Si3(s) + 7SiO(2)(s) on the basis of the most up-to-date thermodynamic values.