Abstract: The thermal expansion of two modifications α- and β-Cs2SO4, as well as the compound Cs2Ca3(SO4)4, was studied for the first time by high-temperature powder X-ray diffraction method in the temperature ranges of 25–960 and 25–540°C, respectively. β-Cs2SO4 transforms into the high-temperature α-Cs2(SO4) modification through a two-phase region in the range of 600–750°C. The thermal expansion of all studied phases is sharply anisotropic: αa = 37.3(10), αb = 36.2(4), αc = 12(5), αV = 85.1(5) at 30°C for β-Cs2SO4; αa = 55(5), αc = 115(9), αV = 224(12) × 10–6 °С–1 at 750°С for α-Cs2SO4. The thermal expansion coefficients for Cs2Ca3(SO4)4 are α11 = 18.8(5), αb = 18.2(5), α33 = –7.5(2), αβ = –10.6(2), αV = 29.6(9) × 10–6 °С–1 at 25°С. The continuity of the polymorphic transformation of Cs2SO4 is shown. It consists in the fact that, with an increase in temperature, the corrugated columns or rods elongated along the c axis in both modifications, consisting of Cs(SO4)6 microblocks, straighten due to the rotation of SO4 tetrahedra. The interpretation of the anisotropy of the thermal expansion of Cs2Ca3(SO4)4 is based on the mechanism of rocking polyhedra. A hinge deformation at the level of Ca(SO4)6 microblocks, which leads to a large negative thermal expansion in the α33 direction, is revealed.