The thermal properties of 1,3,5-trithiane were measured with adiabatic calorimetry and DSC between 15 and 500 K. It was necessary to purify the compound even as the stated purity of the commercial product was >99%. The impurity present triggers a reaction at the melting point. The absolute entropy at 298.15 K was measured to be (156.2 +/- 0.8) J K-1 mol(-1), the enthalpy increment from 0 to 298.15 K is (22936 +/- 100) J mol(-1). The enthalpy of melting was found to be (32.2 +/- 0.3) U mol(-1), the fusion temperature 488.4 K. The molar heat capacity of the solid between 100 and 380 K can be described by a polynomial function C-p = {24.17 + 0.4004T - 9.9605.10(-5)T(2)} J K-1 mol(-1), the standard deviation of this fit is 0.24 J K-1 mol(-1).