The universal occurrence of wear has led to consideration of the wear process from a thermodynamic point of view. The intent was to characterize the nature of the wear process and material behavior by defining the thermo-dynamic conditions under which such processes are possible. The sliding of two surfaces, one of which wears at a much greater rate than the other, was idealized as a single body to which heat and work are supplied and from which mass transfer is allowed. The condition that for an isolated system undergoing some process entropy production must be non-negative was used to develop a constraint for allowable wear processes. This constraint relates heat, work, chemical potential and surface energy and limits the processes and material behavior possible in the model system.