The crystal structure of the boroleucite solid solution Rb _0.40Cs_0.54B_0.94Si_2.06O₆ is refined in space group I-43d by the Rietveld method with the use of the X-ray powder diffraction data. The refinement data complement the available crystal chemical characteristics of Rb_1-x Cs _x BSi ₂O₆ solid solutions. The thermal expansion and phase transformations of Rb_1-x Cs _x BSi₂O₆ borosilicates are investigated in parallel by high-temperature X-ray diffraction with conventional powdered samples and by the dilatometric method with samples in the form of pressed pellets. It is demonstrated that the thermal expansion coefficients, as well as the temperatures and sequence of polymorphic transitions, which are determined from the data obtained by two methods are in close agreement. The temperature curve of the I-43d ⇄ Ia3d phase transition for the Rb_1-x Cs _x BSi₂O₆ solid solution system is constructed from the data obtained by both methods. It is shown with the use of the structural data obtained by the Rietveld method that, at temperatures above 800°C, rubidium-cesium boroleucites undergo decomposition due to the release of alkali cations.