We develop a model to investigate the impact of volume fraction and extent of mixing on the thermal efficiency with which a cavity-based particle suspension solar reactor reduces Co3O4 into CoO and O-2. Thermal efficiency is defined as the fraction of solar energy used to drive the endothermic reduction. In the model, particles move continuously through the reactor in either mixed or plug flow-mixing conditions that give rise to isothermal and nonisothermal suspensions, respectively-and reduce at a rate governed by shrinking core kinetics, the parameters of which are determined using thermogravimetric data. Radiation is simulated using the Monte Carlo Ray Tracing technique. The model is applied to a reactor heated by 4 kW of point-focused solar radiation with a mean concentration ratio of 1400 suns containing monodisperse suspensions of 40 mu m diameter particles with volume fractions between 1 x 10(-5) and 1 x 10(-2). Thermal efficiency is insensitive to mixing for the two conditions considered. The maximum thermal efficiency obtained for mixed flow with an isothermal suspension is 34.1% at 102 g min(-1) and a volume fraction of 2 x 10(-4). At the same volume fraction, the maximum thermal efficiency for nonisothermal plug flow is 33.2% at 94g min(-1). Thermal efficiency is more sensitive to the volume fraction, but only below a threshold value of 2 x 10(-4). Thus, from the perspective of coupling heat transfer to the chemical reaction, design and operational efforts of particle suspension reactors for the reduction of cobalt oxide should focus on generating suspensions of at least this threshold value rather than on mixing the particles within the suspension.