Thermal decomposition of mixed K1 − x Csx BSi2O6 borosilicates by the methods of thermal analysis, annealing, and quenching, with the following refining of the structure by the Rietveld method on the example of solid solutions with x = 0.3 and 0.7 crystallizing in the space groups [TEX equation: I4\bar 3d] and [TEX equation: Ia\bar 3d] , respectively, is studied. It is shown that the solid-phase decomposition of borosilicates proceeds with the release of the gas- eous phase and formation of cristobalite and/or tridymite of SiO2 at the final stage. In this case the solid solutions enriched by potassium decompose in one stage with the formation of SiO2, while the solutions enriched by cesium decompose with the formation of the intermediate zeolite-like CsBSi5O12 borosilicate, which also decomposes during further thermal treatment. According to the data of the structure, the refining of K1 − x Csx BSi2O6 solid solutions obtained by thermal treatment at 1000°C for 20, 65, 80, and 100 h, it is detected that for the samples with x = 0.3 and 0.7 the parameter of the cubic cell of the leucite-like phase decreases and the vacancies in the position of alkaline cation appear. During the solid-phase decomposition of K1 − x Csx BSi2O6 solid solutions the structure degrades.