A nonlinear dynamic model is developed for a steam/methane-reforming reactor that uses concentrated solar radiation as the source of high-temperature process heat. The model incorporates a set of lumped-parameter reservoirs for mass and energy. For each reservoir, the unsteady mass and energy conservation equations are formulated, which couple conduction, convection, and radiation heat transfer with the temperature dependent chemical conversion. Radiative exchange, the dominant heat transfer mode at above 800 K, is solved by a band-approximation Monte Carlo technique. The dynamic model is applied to predict the transient behavior of a 400kW prototype solar reformer in operational modes of purging, thermal testing, startup, chemical reaction, shutdown, and cyclical operation. Time constants vary between 2 s for species transport and 1 x 10(5) s for thermal energy transport through ceramic insulation. Validation is accomplished by comparing modeled and experimentally measured outlet gas temperatures obtained from reactor tests in a solar tower facility. (c) 2007 Elsevier Ltd. All rights reserved.