LuBa3B(9)O(18) borate was prepared using a high-temperature solid-state synthesis. The crystal structure, as in the cases of a- and beta-BaB2O4, is composed of isolated rigid B3O6 groups. For the first time, its thermal behavior was investigated by high-temperature X-ray powder diffraction (HTXRD) within the temperature range of 20-980 degrees C. The LuBa3B9O18 structure expands highly anisotropically. As the temperature increases, the degree of anisotropy decreases (alpha(max)/alpha(min) = 9 at 25 degrees C and 6 at 800 degrees C) and the coefficient of volume expansion increases from 38.9 to 54.1 x 10(-6) C-1, respectively. A comparative analysis of the nature of thermal deformations of the three borates under consideration was conducted.