Solar-thermal reactors make use of concentrated solar irradiation to drive endothermic reactions. One example of processes that can be carried out in solar-thermal reactors is the solar-thermal conversion of biomass into synthesis gas (H-2, CO and CO2). The operation of these reactors is dependent on available sunlight and is affected by the presence of clouds, which act as disturbances. During a partly cloudy day, operation of solar-thermal reactors is still possible but flow rate adjustment is required in order to maintain consistent operation. Thus, a robust control system that will allow continuous high performance operation of the reactor is required in order to make the process financially viable. A model predictive control system (MPC) is proposed, which uses a model of the process to determine the required control signal. The first step in the development of the control system is the formulation of a dynamic mathematical model that describes process behavior adequately yet can be solved in real time. A simplified dynamic model for a solar-thermal transport-tube reactor has been developed based on unsteady mass and energy balances. The model was solved using MATLAB (TM) and validated with experimental data. Model validation was carried out at the High Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL) in Golden, CO, using different solar power level inputs. In this work, a description of the model and a comparison with the experimental results are presented. Simulations of the model were performed for a reflective and an absorbing cavity. The former allows for faster heating during experimental runs, while the latter is more representative of an industrial setting. (C) 2012 Elsevier B.V. All rights reserved.