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The hydrated potassium hemimagnesium dihydrogen pyrophosphate KMg0.5H2P2O7·H2O was synthesized. It crystallizes in the triclinic system, space group P1¯ (n. 2), Z=2, with the following unit-cell parameters: a=6.8565(2) A˚, b=7.3621(3) A˚, c=7.6202(3) A˚, α=81.044(2)°, β=72.248(2)°, γ=83.314(3)°, V=360.90(2) A˚3. The structure was obtained by single-crystal X-ray diffractometry, and a full-matrix least-squares refinement based on F2 gave a final R index of =0.0368 (wR=0.0975), utilizing 1446 observed reflections with I>2σ(I). The crystal packing consists in a three-dimensional network made by layers parallel to ab plane of PO4 double tetrahedra and MgO6 octahedra, linked by hydrogen bonds, while K atoms form complex coordination within cavities between tetrahedra and octahedra. The dihydro-pyrophosphate anion (H2P2O7)2− shows bent eclipsed conformation and the Mg2+ ion lies on inversion center. No coincidences observed between most of infrared and Raman spectral bands confirmed the centrosymmetric structure of the title compound; the vibrational spectra point to a bent POP bridge angle. [Copyright &y& Elsevier]