A mathematical description of thermal drying process involving various mechanisms of moisture migration such as water sorption, diffusion and solid-moisture interactions in hygroscopic materials is developed for unsaturated flow region. The model has been formulated using microscopic mass balance and volume averaging techniques. The constitutional laws such as Darcy’s law are modified to account for solid-moisture interactions. Effective diffusion coefficients were estimated using moisture isotherms for three distinct systems of differing hygroscopicity. Drying curves were simulated using the estimated effective diffusion coefficient for potato, and compared with the experimental data. The analysis demonstrates the strong dependence of the effective diffusion coefficient on the shape of the moisture isotherm.