A new vanado-molybdate LiMg3VMo2O12 has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li2-zMg2zVzMo 3-zO12 was investigated and the structures of the z=0.5 and 1.0 compositions were refined by Rietveld analysis of powder X-ray (XRD) and powder neutron diffraction (ND) data. The structures were refined in the orthorhombic space group Pnma with a∼5.10, b∼10.4 and c∼17.6 , and are isostructural with the previously reported double molybdates Li 2M2(MoO4)3 (M=M2, z=0). The structures comprise of two unique (Li/Mg)O6 octahedra, (Li/Mg)O6 trigonal prisms and two unique (Mo/V)O4 tetrahedra. A well-defined 1:3 ratio of Li+:Mg2 is observed in octahedral chains for LiMg3VMo2O12. Li+ preferentially occupies trigonal prisms and Mg2 favours octahedral sheets. Excess V5 adjacent to the octahedral sheets may indicate short-range order. Ionic conductivity measured by impedance spectroscopy (IS) and differential scanning calorimetry (DSC) measurements show the presence of a phase transition, at 500600 °C, depending on x. A decrease in activation energy for Li+ ion conductivity occurs at the phase transition and the high temperature structure is a good Li+ ion conductor, with σ=1×10-34×10-2S cm -1 and Ea=0.6 to 0.8 eV.