In computational materials science, the phase-field method is a well-established method for simulating microstructure evolution [1]. It allows a numerically efficient tracking of material surfaces due to the associated order parameter. Its evolution equation is commonly derived by a variational approach or a corresponding principle of virtual power. Both approaches consider the order parameter as additional degree of freedom and assume a diffuse interface from the outset. Here, following the discussion of [2], the order parameter is introduced as an internal state variable instead of an additional degree of freedom. In addition, the phase-field method is considered as an approximation of the sharp interface theory of a continuum containing a singular surface [3]. In the context of continuum thermodynamics, the evolution equation of the order parameter is consistently derived on the basis of the Clausius-Duhem inequality. Thereby, the heat conduction, the thermomechanical coupling, and the role of latent heat due to phase evolution are discussed for the diffuse interface region. The presented approach is compared with the classical variational approach in terms of the evolution equation. REFERENCES [1] B. Nestler and H. Garcke and B. Stinner, Physical Review E, Vol. 71, No. 4, pp. 041609 1–6, 2005. [2] G. A. Maugin, Mech. Res. Commun., Vol. 69, pp. 79–86 , 2015. [3] A. Prahs, T. Böhlke, Contin. Mech. Thermodyn, Vol. 32, pp. 1417–1434, 2019.