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Much attention has been paid to experimental investigation of the thermodynamic properties of aromatic and heteroaromatic compounds since the beginning of the 20th century. The available data on thioxanthone (THX), sulfur-containing heterocycle, is in a significant scatter. The phase transition enthalpies of THX were reconciled by combining conventional measurements and empiric approaches recently developed in our laboratory. The heat capacities in the crystal and liquid states were determined by adiabatic calorimetry and power-compensated differential scanning calorimetry. The fusion enthalpy at the melting temperature was measured and adjusted to 298.15 K according to Kirchhoff’s law of thermochemistry. The value at 298.15 K was found using solution calorimetry. From these data, the temperature dependence of the sublimation enthalpy was evaluated. Ideal gas-phase heat capacities were calculated applying rigid rotor–harmonic oscillator approximation and then used to independently derive the temperature dependence of the sublimation enthalpy.