Lithium phyllomanganate, LixMnO2 . yH(2)O (x = 0.25-0.29, y approximate to 0.6) with the birnessite structure cannot be obtained directly by oxidation of Mn(OH)(2) in Li-containing medium. It was obtained by a two-step topotactic reaction involving an acidic treatment of sodium phyllomanganate, followed by ion exchange in LiOH at room temperature. The lamellar lithium manganate is characterized by electron diffraction, magnetic measurements and thermogravimetry, showing that the structure is retained after water loss up to 500 degrees C. The material cannot be chemically oxidized by bromine, cerium(IV), or nitryl fluoborate. However, sodium and lithium phyllomanganates can be delithiated reversibly in electrochemical cells at ca. 4 V vs. Li/Li+.