We report here the direct synthesis of the hexagonal form of sodium manganese dioxide using the mild hydrothermal decomposition of sodium permanganate in water at 170 degrees C. Sodium manganate, Na0.35MnO2 . 0.7H(2)O, has an R (3) over bar m rhombohedral structure like Li-x(H2O)TiS2 with a 7 Angstrom repeat distance indicative of a monolayer of water between the manganese dioxide layers. The water may be readily removed by gently heating to 150 degrees C in air. This manganese oxide and a new vanadium oxide, also formed hydrothermally, react readily and reversibly with lithium ions and are under evaluation for use as the cathode in high energy density lithium batteries.