Abstract Single crystals of ra-Mg2CuV2O8 (I) and MgCu2V2O8 (II) have been prepared by solid state reactions. The orange (I) and dark red (II) crystals show monoclinic symmetry, space group C52h -P21/c, (I): a = 6.167(3), b = 8.172(5), c = 6.400(8) Å, β = 116.22(3)°, Z = 2; (II): a = 6.453(1), b = 8.342(2), c = 11.517(2)Å,β 90.44(2), Z = 4. Both compounds show ordered metal distributions and vanadium in tetrahedral coordination by oxygen. (I) is characterized by elongated CuO6 octahedra and (II) by trigonal CuO5 bipyramids. m-Mg2CuV2O8 is the low temperature form of the formerly investigated orthorhombic o-Mg2CuV2O8. Calculations of the Coulomb parts of the lattice energy of the two crystal forms are discussed