An approximate theoretical enthalpy model is developed to study the phase-change process in porous media. During the melting or the freezing process, the interface within a pore remains at the phase-change temperature until the process is completed. Since the melting process is relatively slow, the assumption of local thermal equilibrium is not universally valid. This theoretical study leads to the development of working relations. An approximate two-temperature model is studied analytically. The results provide the parametric information concerning the phase-change front. Also, the conditions that would assure the existence of local thermal equilibrium are presented. (C) 2001 Elsevier Science Ltd. All rights reserved.