An analysis of the melting process for a phase change material in which the solid density is greater than that of the liquid is presented. The mathematical model developed investigates the effect of unequal densities on the shape and location of the solid-liquid interface for a system within a horizontal cylindrical enclosure. This model offers a solution for the coupled conduction and interface balance equations for the three problems of; constant wall temperature, constant heat flux, and constant heat transfer coefficient imposed at the cylinder surface. The numerical method of finite difference is used to solve the system of governing equations. In the solution of the conduction equation, the implicit alternating direction technique is implemented. It is proposed that the heavier solid phase drops to the base of the cylinder as it melts, and maintains a position antisymmetric with respect to the cylindrical axis. To check the validity of this, experimental tests are made in which the interface position is photographed at selected time intervals. These photographs indicate that the mathematical model is physically justified.