A solar receiver-reactor concept for high-temperature thermochemical applications involving gas and condensed phases is presented. It features two cavities in series. The inner cavity is an enclosure, e.g., made of graphite, with a small aperture to let in concentrated solar power. It serves as the solar receiver, radiant absorber, and radiant emitter. The outer cavity is a well-insulated enclosure containing the inner cavity. It serves as the reaction chamber and is subjected to thermal radiation from the inner cavity. The advantages of such a two-cavity reactor concept are outlined. A radiation heat transfer analysis based on the radiosity enclosure theory is formulated and results are presented in the form of generic curves that indicate the design constraints. High energy absorption efficiency can be achieved by minimizing the aperture area, by maximizing the size of the inner cavity and by constructing it from a material of high emissivity.