Borates / Crystal structure at high temperature / Rigid borate group / Thermal expansion / High-temperature single-crystal and powder X-ray diffraction Abstract. The paper presents a brief review of the pre-sent state of high-temperature borate crystal chemistry. This review summarizes the results of high-and low-tem-perature single crystal X-ray diffraction studies for more than 10 borate structures and high-temperature powder X-ray diffraction data for about 65 borates. Thermal behavior of their crystal structures, thermal expansion, polymorphic transitions and their relationship to borate glasses are pre-sented. These studies allow to formulate the basic princi-ples of high-temperature borate crystal chemistry and to reveal the regularities of thermal behavior of borates. On heating, the BO 3 and BO 4 polyhedra and rigid groups con-sisting of these polyhedra, practically maintain their con-figuration and size, but they are able to rotate like hinges exhibiting highly anisotropic thermal expansion, including linear negative expansion. Based on these results, we gen-eralize the term "rigid group" and render thermal vibra-tions as the key ingredient for the self-assembly of borate rigid groups.