Anhydrous alkali copper sulfates constitute the most abundant group of mineral species from the Second Scoria Cone of the Great Tolbachik Fissure Eruption (1975–1976), a location being renowned for its great mineral diversity. Euchlorine, ideally KNaCu ₃O(SO ₄) ₃, is the prevalent mineral in the hot sulfate-rich zones of the fumaroles. In this work, its thermal expansion and hydration/dehydration behavior have been studied. The results of a structure refinement from new single-crystal diffraction data are also reported, and a description of the structure based on anion-centered coordination polyhedra is given. The strongly anisotropic character of the thermal expansion of euchlorine remains essentially unchanged up to its decomposition. The strongest α ₁₁ expansion is observed approximately perpendicular to the alkali interlayer of the structure, whereas the minimal α ₂₂ and α ₃₃ thermal expansion coefficients are parallel to the plane of {Cu ₃O(SO ₄) ₃} ²⁻ layers. Hydration experiments controlled by X-ray powder diffraction reveal a very complex behavior with multicomponent phase formation. Remarkably, upon heating stepwise dehydration occurs, whereby the complex mixture of hydrated sulfates gradually reverses and becomes again essentially single-phased anhydrous euchlorine.