  The thermal relaxation of a dense gas described by the modified Enskog
equation is studied for a closed system in contact with a heat bath. As in the
case of the Boltzmann equation, the Helmholtz free energy $\mathcal{F}$ that
decreases monotonically in time is found under the conventional kinetic
boundary condition that satisfies the Darrozes--Guiraud inequality. The
extension to the modified Enskog--Vlasov equation is also presented.
