  We study the thermodynamic behavior of a decaying scalar field coupled to a
relativistic simple fluid. It is shown that if the decay products are
represented by a thermalized bath, its temperature evolution law requires
naturally a new phenomenological coupling term. This ``energy loss'' term is
the product between the enthalpy density of the thermalized bath and the decay
width of the scalar field. We also argue that if the field $\phi$ decays
"adiabatically" some thermodynamic properties of the fluid are preserved. In
particular, for a field decaying into photons, the radiation entropy production
rate is independent of the specific scalar field potential $V(\phi)$, and the
energy density $\rho$ and average number density of photons n scale as $\rho
\sim T^{4}$ and $n \sim T^{3}$. To illustrate these results, a new warm
inflationary scenario with no slow roll is proposed.
