  We consider the Lema\^{\i}tre-Tolman-Bondi metric with an inhomogeneous
viscous fluid source satisfying the equation of state of an interactive mixture
of radiation and matter. Assuming conditions prior to the decoupling era, we
apply Extended Irreversible Thermodynamcs (EIT) to this mixture. Using the full
transport equation of EIT we show that the relaxation time of shear viscosity
can be several orders of magnitude larger than the Thomson collision time
between photons and electrons. A comparison with the ``truncated'' transport
equation for these models reveals that the latter cannot describe properly the
decoupling of matter and radiation

General Relativity and Quantum Cosmology;Astrophysics;Statistical Mechanics;Plasma Physics