The discovery of numerous endemic anhydrous sulfate minerals in fumaroles of the Tolbachik volcano (Kamchatka, Russia) has revived interest in the whole family of anhydrous sulfates. Herein is reported the crystal structure of Cs₂Cu(SO₄)₂which adds important data on the 'final' contributor with the largest A ⁺cation to the A₂[Cu(SO₄)₂] morphotropic series (A = Na, K, Rb, Cs), the 'initial' structurally characterized representative of this family being saranchinaite Na₂Cu(SO₄)₂. With increasing ionic radius of the alkali metal cation(s), embedded in the [Cu(SO₄)₂]^2-framework, symmetry-breaking transformations occur. Cs₂Cu(SO₄)₂, which is here designated as the ϵ-phase, has a layered structure. Cs₂Co₂(SO₄)₃is a new representative of another morphotropic series of the orthorhombic A 2[M ²⁺2(SO₄)₃] family, being also the first anhydrous Cs-Co sulfate. Structural relationships in A ⁺2 M ²⁺(SO₄)₂and A ⁺2 M ²⁺2(SO₄)₃morphotropic series are discussed in detail.