The thermal expansion of anhydrite, CaSO4, has been measured from 22?? to 1,000?? C by X-ray diffraction, using the Guinier-Lenne?? heating powder camera. The heating patterns were calibrated with Guinier-Ha??gg patterns at 25?? C, using quartz as internal standard. Heating experiments were run on natural anhydrite (Bancroft, Ontario), which at room temperature has lattice constants in close agreement with those of synthetic material. The orthorhombic unit cell at 22?? C (space group Amma) has a=7.003 (1) A??, b=6.996 (2) A?? and c=6.242 (1) A??, V=305.9 (2) A??3. At room temperature, the thermal expansion coefficients ?? and ?? (?? in ??C-1??104, ?? in ??C-2??108) are for a, 0.10, -0.69; for b, 0.08, 0.19; for c, 0.18, 1.60; for V, 0.37, 1.14. Second-order coefficients provide an excellent fit over the whole range to 1,000?? C. ?? 1979 Springer-Verlag.