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The title compound, diiron(III) trisulfate–sulfuric acid–water (1/1/28), has been prepared at temperatures between 235 and 239 K from acid solutions of Fe2(SO4)3. Studies of the compound at 100 and 200 K are reported. The analysis reveals the structural features of an alum, (H5O2)Fe(SO4)2·12H2O. The Fe(H2O)6 unit is located on a centre of inversion at (inline image, 0, inline image), while the H5O2+ cation is located about an inversion centre at (inline imageinline imageinline image). The compound thus represents the first oxonium alum, although the unit cell is orthorhombic.