Thermal explosion theory is applied to a system comprising one chemically reacting medium embedded in another. The model makes use of Semenov's treatment. The Arrhenius part of the heat production term is simplified by means of the exponential approximation, and because of the assumption of spatially uniform temperatures inherent in Semenov's formalism, thermal boundaries are characterized by temperature steps. The treatment is extended to the case where one of the media reacts endothermically. The possibility of stabilization of a core reaction by an outer endothermic one is investigated.