Unsteady radiative heat transfer is analyzed numerically in a directly-irradiated planeparallel medium containing a suspension of ceria particles undergoing non-stoichiometric thermal reduction. The micrometer-sized ceria particles are assumed to be homogenous, nongray, absorbing, emitting, and anisotropically scattering, while the overall medium is of nonuniform temperature and composition. The unsteady mass and energy conservation equations are solved using the finite-volume method and the Shampine-Gordon time integration scheme. Radiative transport is modeled using the energy-portioning Monte Carlo ray-tracing method with radiative properties obtained from the Mie theory. Increasing particle volume fraction and decreasing particle diameter both increase the optical thickness of the particle suspension, resulting in increasing peak temperature and non-stoichiometry at steady state.For 5 µm-dia. particles under 1000 suns irradiation, the peak temperature at steady state