A solid is bounded by two infinite parallel plates and the whole system is at a temperature TA which is below the freezing point Tf of the substance. At a certain instant, one of the plates, B, is raised to and maintained at a temperature TB which is above the freezing point Tf, while the other plate A is maintained at the original temperature TA such that TA<Tf<TB. The problem of finding the temperature distribution in the solid and the liquid phases and the position of the interface between the two phases as functions of time is solved by a method in which a solution of the heat conduction problem is constructed by the superposition of solutions such that all the initial, asymptotic and boundary conditions of the problem are satisfied.