This paper aims to theoretically explain the behavior of thermal diffusion coefficient in polymer solutions; specifically, in the vicinity of the unstable regions of phase diagrams. Based on an equation that was derived in the framework of classical irreversible thermodynamics, a dimensionless number was introduced that plays a crucial role in determining the thermal diffusion in polymer solutions. The behavior of thermal diffusion coefficient in various phase diagrams was investigated by changing the Flory-Huggins interaction parameter to include concentration and temperature dependencies using Koningsveld interaction factor. It was shown that irrespective of the types of phase diagrams, the proposed equation for thermal diffusion becomes zero at infinite dilution and takes finite values on the spinodal line. Moreover, it was illustrated that in a binary polymer solution the Soret coefficient (the ratio of the thermal diffusion coefficient to the mutual diffusion coefficient) diverges at the critical point, and more importantly it was predicted that the divergent behavior of the Soret coefficient is observed at all the points of the spinodal line. (C) 2016 Published by Elsevier Ltd.