Stoichiometric orthorhombic O-LiMnO 2 , prepared at high temperature from a mixture of Mn 2 O 3 and LiOH.H 2 O, is electrochemically active in a lithium deintercalation/intercalation process. The samples with the smallest grains ( < 1 μm), and corresponding to the phases prepared with some lithium hydroxide deficiency developed the higher weight capacities. These could reach up to 160 mAh/g under a C/15 regime following a forming that spanned over about thirty cycles. The cells capacity remained constant after forming, showing the very good cyclability of the system. It has not been clearly demonstrated whether the Li/Mn substitution in the O-LiMnO 2 samples played an important role in the electrochemical characteristics of the Li/Li x MnO 2 cells, although the group with small grains also showed, on average, a higher cationic disorder. A phase transition took place upon the first oxidation of O-LiMnO 2 . The new phase presented an electrochemical behavior resembling that of spinel LiMn 2 O 4 . A modeling of the discharge curves showed a progressive forming of the materials during a few cycles.