Unsteady radiative heat transfer is analyzed numerically in a directly irradiated plane-parallel medium containing a suspension of ceria particles undergoing nonstoichiometric thermal reduction. The micrometer-sized ceria particles are assumed homogenous, nongray, absorbing, emitting, and anisotropically scattering, whereas 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 the particle volume fraction and decreasing the particle diameter both increase the optical thickness of the particle suspension, resulting in increasing peak temperature and nonstoichiometry at steady state. For 5 mu m-diam particles under 1000 suns irradiation, the peak temperature at steady state ranges from 1855 K for a particle volume fraction of f(v) = 10(-6) to 2092 K for f(v) =10(-4); the temperature nonuniformity ranges from 9 to 622 K. For a fixed volume fraction of f(v) = 10(-6), decreasing the particle diameter from 20 to 1 mu m increases the peak temperature at steady state from 1734 to 2162 K; the temperature nonuniformity increases from 9 to 61 K.