The crystal chemistry of Rb-Cs boroleucites has been studied by means of X-ray powder diffraction at room and elevated temperatures. The cubic I-43d → cubic Ia3d phase transition was investigated using a series of samples prepared by solid-state reaction along the pseudobinary system RbBSi ₂O₆ - CsBSi₂O₆. The Rietveld refinement of the structures of Rb_1-xCs_xBSi ₂O₆ solid solutions (x = 0.2, 0.4, 0.6, 0.8) demonstrates that the solutions with a high Rb content crystallise in the cubic I-43d space group, and the boroleucites with a considerable Cs content have Ia3d symmetry. Rb can substitute Cs in a wide range of compositions. Within a narrow range of x = 0.5 - 0.6 immiscibility was revealed. Under Rb-Cs substitution the cubic lattice parameter, the (Rb,Cs)-O distances, and the angles between tetrahedra of the I-43d phase change clearly, while those of the Ia3d phase change slightly. The HTXRD data shows that the I-43d phase transforms into a Ia3d phase on heating analogously to a change of the composition. As the Cs content increases the transition temperature decreases. The low temperature 1-43d phase shows a higher thermal expansion than the high temperature Ia3d phase.