The heat capacity of a very pure sample of xenon tetrafluoride, XeF4, was determined by adiabatic calorimetry between 5 and 350°K. No irregular thermal behavior was observed, except in the region near 210°K, where slightly enhanced heat capacity values, ascribed to a small amount of impurity, were observed. The values of the thermodynamic quantities Cp°, S°, (H° − H °0)/T, and (G° − H °0)/T at 298.15°K are 118.39 ± 0.12, 167.00 ± 0.17, 77.24 ± 0.08, and −89.76± 0.09 J °K−1 · mole−1, respectively. The standard entropy of the gas at 298.15°K was calculated from the thermal data to be 323.2± 2.0 J °K−1· mole−1, in excellent agreement with the value 323.98 ± 0.4 J °K−1· mole−1 calculated from electron-diffraction data and the frequency assignment of Tsao, Cobb, and Claassen. The standard entropy of formation of solid XeF4 at 298.15°K was found to be −407.95 ± 0.17 J °K−1 · mole−1, and the standard Gibbs energy of formation at 298.15°K is −145.48 ± 0.88 kJ mole−1. The corresponding quantities for gaseous XeF4 at 298.15°K are −250.97 ± 0.40 J °K−1· mole−1 and −131.36 ± 0.91 kJ mole−1. The standard enthalpies of formation of the solid and of the gas at 0°K are −266.29 ± 0.88 kJ mole−1 and −201.39 ± 0.90 kJ mole−1, respectively. The equilibrium constant for the formation of gaseous XeF4 from the elements at various temperatures is tabulated and compared with the experimental measurements by Weinstock, Weaver, and Knop.