A new vanado-molybdate LiMg 3 VMo 2 O 12 has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li 2−z Mg 2+z V z Mo 3−z O 12 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∼17.6Å, and are isostructural with the previously reported double molybdates Li 2 M 2 (MoO 4 ) 3 (M=M 2+ , z=0). The structures comprise of two unique (Li/Mg)O 6 octahedra, (Li/Mg)O 6 trigonal prisms and two unique (Mo/V)O 4 tetrahedra. A well-defined 1:3 ratio of Li + :Mg 2+ is observed in octahedral chains for LiMg 3 VMo 2 O 12 . Li + preferentially occupies trigonal prisms and Mg 2+ favours octahedral sheets. Excess V 5+ 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 500–600°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 −3 –4×10 −2 Scm −1 and E a =0.6 to 0.8eV.