Borosulfates provide fascinating structures and properties that go beyond a pure analogy to silicates. Mg-3[H2O -> B(SO4)(3)](2) is the first borosulfate featuring a boron atom solely coordinated by three tetrahedra. Thus, the free Lewis acidic site forms a Lewis acid-base adduct with a water molecule. This is unprecedented for borosulfate chemistry and even for borates. Quantum chemical calculations on water exchange reactions with BF3 and B(C6F5)(3) revealed a higher Lewis acidity for the borosulfate anion. Moreover, proton exchange reactions showed a higher Bronsted acidity than comparable silicates or phosphates. Additionally, Mg-3[H2O -> B(SO4)(3)](2) was characterised by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and density functional theory (DFT) calculations.