Lithium manganese (II) vanadate dihydrate, Li2Mn(VO3)(4) . 2H(2)O, has been prepared hydrothermally and characterized structurally from single-crystal diffraction data. It crystallizes in space group C2/m of the monoclinic system with Z = 2 in a cell of dimensions a=19.286(5) Angstrom, b=3.523(1) Angstrom, c=8.164(2) Angstrom and beta =105.38(3)degrees. The structure consists of [Mn(VO3)(4)](2-) layers, built up from MnO6 octahedra and VO5 square pyramids, separated by Li+ ions and water molecules. If the discharge rate is slow enough, Li2Mn(VO3)(4). 2H(2)O can insert reversibly up to 2 Li per formula unit between OCVs of 2.6 V and 2.15 V, while a new, unidentified, phase is obtained after insertion of 4 Li. Upon oxidation, lithium deinsertion occurs at high voltage (above 4.4 V), leading to LixMn(VO3)(4) . 2H(2)O materials with x less than or equal to 0.7. It is hidden by an important electrolyte oxidation. The Mn3+/Mn2+ redox couple, associated to the lithium deinsertion/insertion process, was estimated at a high voltage value (between 3.5 and 4.5 V), due to the inductive effect of V-V on the Mn-O bonds. O 2000 Elsevier Science B.V. All rights reserved.