We model the decomposition reaction of a solid powder into a solid plus gas in a mixed homogeneous open solar reactor and derive criteria for stability. Instability may occur via a feedback mechanism, whereby gas produced in the decomposition reaction reduces the inflow, which leads to even higher reaction rate. For a variety of reactions proposed for solar application, we list the limits of stable operation as the maximum fraction of solid educt in the input carrier gas both for complete as well as for partial decomposition.