The thermodynamic phase behavior of a polymer–diluent system (atactic polystyrene–1-dodecanol) forms the fundamental basis of the description of thermally-induced demixing processes. In this paper, we demonstrate that temperature modulated differential scanning calorimetry (TMDSC) can accurately detect the liquid–liquid demixing transition. This transition can be clearly observed in the modulus of the complex heat capacity signal and in the phase angle. The phase angle shift is very small during liquid–liquid demixing so liquid–liquid demixing of a polymer–diluent system takes place at time scales instantaneously in comparison with the modulation period of TMDSC. In addition, the glass transition temperature of the polymer-rich phase and the crystallization temperature of the diluent can be determined as well within the same TMDSC experiment.