The reheating process in inflationary universe models is considered as
an out-of-equilibrium mixture of two interacting and reacting fluids,
and studied within the framework of causal, irreversible thermodynami
cs. The evolution of the temperature and the decay rate as determined
by causal thermodynamics are estimated at different stages of the proc
ess. A simple model is also used to find the perturbations of the expa
nsion rate, including the possibility of damped oscillations.