The crystal structures of two K_1-xCs_xBSi₂O₆ solid solutions have been refined at room temperature dy the Rietveld method: x = 0.12, a = 12.6858(4) Å, R_wp = 7.66%, R_F = 5.56% and x = 0.50, a = 12.8480(2) Å, R_wp = 7.64%, R_F = 3.10%. They are isostructural to cubic KBSi₂O₆ with the space group I4̄3d. The structure is built up from (Si,B)O₄ tetrahedra linked in four-, six- and eight-fold rings which are forming a three-dimensional borosilicate framework. The framework contains large cavities that are placed in continuous channels along the [111] directions. The Cs and K atoms occupy the positions in the channels statistically. Thermal behaviour of KBSi₂O₆ and K_0.5Cs_0.5BSi₂O₆ has been studied by high-temperature powder X-ray diffraction within the temperature range of 293-1073 K. A new tetragonal polymorph of KBSi₂O₆ has been found in situ under heating. The new polymorphic I4̄3d (cubic) - Ia3̄d (cubic) transition and the new Ia3̄d cubic polymorphic phase has been proposed for K_1-xCs_xBSi₂O₆ from our experimental and literature data on crystal structures and thermal expansion of leucites. The structural relaxation under cationic (K, Cs) substitutions and under heating has been investigated.