  We investigate divergence-type theories (DTT) describing the dissipative
interaction between a field and a fluid. We look for theories which, under
equilibrium conditions, reduce to the theory of a Klein-Gordon scalar field and
a perfect fluid. We show that the requirements of causality and positivity of
entropy production put non-trivial constarints to the structure of the
interaction terms. These theories provide a basis for the phenomonological
study of the reheating period.
