The high-temperature structural behaviour of the monoclinic and cubic phases of ANA-type boroleucite, KBSi2O6, was studied by X-ray powder diffraction. Boroleucite crystallizes with different symmetry depending on the method of preparation: the P21/a monoclinic modification was obtained by dry high-pressure synthesis at 600 °C and 0.5 GPa, whereas the cubic I3d phase was prepared by solid-state reaction at 700 °C, 1 bar, for 72 h. Both modifications undergo reversible transformations at higher temperatures, viz. P21/a ↔ (at 800 °C) Iad and I3d ↔ (at 300 °C) P21/a ↔ (at 550 °C) Iad, respectively. The values of the thermal-expansion coefficients are reported. The high-temperature Iad modification of KBSi2O6 exhibits the lowest thermal expansion (6–10 × 10−6 °C−1); it decomposes above 1000 °C with formation of SiO2.