The crystal structure of Cs_0.82B_1.09Si _1.98O₆ boropollucite at room temperature was determined by direct methods and refined in the Ia3̄d space group using an anisotropic approximation of atomic thermal displacements (a=13.009 (1) Å, Z=16, R_w=0.027, R_F=0.037 for 141 independent observed (F≥4σ_F) reflections). The occupancy factors have been refined for Cs and tetrahedral positions assuming the oxygen sites being fully occupied. The compound is isostructural to leucite-pollucite high-temperature modification. Thermal behaviour of CsBSi₂O₆ was investigated by DTA and TG, annealing at different temperatures with following wet chemical analysis and high-temperature X-ray powder diffraction methods. The CsBSi₂O₆ cubic phase loses mass before melting and decomposes to form a new crystalline phase with close to CsBSi₃O ₈ stoichiometry in the temperature range of 1303-1353 K. Thermal expansion of two boropollucite samples, which differed in the number of Cs ⁺ and/or B³⁺ ions of a nominal composition CsBSi ₂O₆ was investigated in air as well as in vacuum. Temperature ranges of negative thermal expansion were found.