According to the high-temperature X-ray powder diffraction data, lamprophyllite and barytolamprophylhte are stable up to 925 and 850 °C, respectively. The temperature of decomposition was defined for barytolamprophyllite by DSC and TGA methods as 897 °C. Analysis of the X-ray diffraction pattern during the heating has not detected any significant changes, implying the absence of phase transitions. Behavior of lamprophyllite and barytolamprophylhte at high-temperature is strongly anisotropic, thermal expansion coefficients (%10 ⁶ ) along crystallographic axes at T = 200 °C are as following: a _a = 12.6(6), a _b = 7.7(1), a _c = 4.4(2) °C-1 for lamprophyllite, and a _a = 11.8(5), a _b = 5.7(1), a _c = 3.4(3) °C-1 for barytolamprophylhte. At T = 600 °C these values are a _a = 15.2(5), a _b = 7.7(1), a _c = 2.2(2) °C-1 for lamprophyllite, and a _a = 12.3(7), à _b = 5.7(1), a _c = -3.8(4) °C-1 for barytolamprophylhte. The contraction along the axis ñ was observed for barytolamprophyllite, but did not for lamprophyllite; it is caused by thermal oxidation of iron.