Mg2.56V1.12W0.88O8 crystals were grown from a MgO/V2O5/WO3 melt, X-ray single-crystal diffraction studies revealed that it is orthorhombic with space group Pnma, a = 5.0658(5) angstrom, b = 10.333(1) angstrom, c = 17.421(2) angstrom, Z = 6, and is isostructural with Mg2.5VMoO8. Raman spectra are reported, and the assignment of the Raman bands is made by comparing the metal-oxygen vibrations of VO4/WO4 tetrahedra in Mg2,5VWO8 with the metal-oxygen vibrations Of VO4/MoO4 tetrahedra in Mg2.5VMoO8. The stretching vibrations appearing at 1016 and 1035 cm(-1) are assigned to Mo=O and W=O double bonds, respectively, associated with the Mg2+ cation vacancies.