We describe a methodology for the initial development of solar thermochemical reactors for converting concentrated solar energy into chemical fuels. It consists of determining the implications that the thermodynamics and kinetics of the chemical transformation have on the initial reactor design. The method is applied for a specific case study: the decomposition of iron oxide above 1875 K, as part of a two-step thermochemical cycle for producing hydrogen from water. We demonstrate that the chemistry of the reaction places important constraints on various engineering design aspects, and we present two reactor concepts that satisfy these constraints. This study addresses the initial steps necessary for the design and development of solar chemical reactors.