The study deals with a dynamic modeling of a solar thermochemical reactor operating continuously to simulate its behavior during transient periods. This reactor is devoted to the thermal reduction of volatile metal oxides which are involved in water-splitting cycles for hydrogen production. Unsteady mass and energy balances are solved to deter-mine the evolution of the reactor temperature and of the outlet gas composition versus time. The kinetics of the chemical reaction is considered in the specific case of zinc oxide dissociation for which reliable data are available. For the chosen reactor design, the thermal inertia of the reactor materials has a weak influence on zinc production during short solar flux interruptions. Energy losses by conduction through reactor walls are the highest at small scale (ranging between 30% and 40% at 1 kW scale), whereas radiative losses through the aperture become predominant at large scale (50 MW scale) and greatly depend on the solar concentration ratio. Then, simulations show that a minimum concentration ratio of 2500 is necessary to reach a sufficient temperature (above 2000 K) allowing efficient ZnO dissociation. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.