About the role of constraints in the linear relaxational behaviour of thermodynamic systems

A formalism is presented by which the linear relaxational behaviour of thermodynamic systems can be described. Instead of using the concept of internal variables of state a set of so-called constraint equations is introduced. These equations represent structural properties of the system and turn out to be related to the shape of the relaxation and retardation spectra of the system. Furthermore some problems concerning the concept of internal variables of state are clarified.