Orthorhombic BaB8O13 (a = 8.550, b = 17.352, c= 13.211 Å, D = 2.927 g/cm3 at 25 °C) transforms reversibly at 700 °C to a high-temperature tetragonal form (a = 8.629, c = 13.252 Å, D = 2.906 g/cm3) stable from 700 °C to the congruent melting point (889 °C) of the compound. The transition is rapid and probably displacive in character. At the transformation point cell constants change discontinuously, doubling of the b axis is lost and cell contents are reduced from Z = 8 to Z = 4. Doubling of the b axis reappears with cooling to the transformation temperature and volume strain is relieved by formation of multiple twins or domains. The latent heat of transformation is 2100 J/mol (0.50 kcal/mol) and dT/dP = -0.0363 K/bar.