In this work, two polymorphic forms of 2,2'-thiodiethylene bis[3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate] (abbreviated as TBHP) were successfully isolated, identified, and characterized by using powder X-ray diffraction and differential scanning calorimetry. It was found that form I has a lower melting temperature than, form II. The solubility data of both form I and form II of TBHP in six pure solvents were experimentally measured in the temperature range of '(283.15 to 318.15) K at atmospheric pressure by using a dynamic:Method. For all of the tested solvents, the solubility data Of TBHP form I are higher than those of form II. The modified Apelblat equation was used to correlate the solubility of TBHP forth. I and form II. The mixing Gibbs energy, the mixing enthalpy, and the mixing entropy of both forms were also determined. It was also found that mixing processes of both forms are endothermic, entropy-driven, and spontaneous. Combining the DCS data and all of the thermodynamic data, it was concluded that the relationship between form I and form II of TBHP is monotropic.