LiFeO2-Li2MnO3 solid solution was synthesized using solid-state reaction and hydrothermal-postannealing methods and characterized as a positive electrode material for rechargeable lithium batteries. Although the maximum Fe content [Fe/(Fe + Mn)] was limited up to 30% by solid-state reaction, the content can extend up to 75% by the hydrothermal-postannealing method. Neutron and X-ray Rietveld analysis reveal that the basic structure of the sample is a layered rock-salt structure isostructural with LiCoO2 (R (3) over barm) in which Fe ions exist on both Li (3a) and Co (3b) sites. Elemental analysis and Fe-57 Mossbauer spectra show Fe ions exist as 3+/4+ mixed-valence state after the samples were postannealed above 650degreesC. The initial charge capacity of Li/sample cells was above 100 mAh/g when the upper voltage limit was 4.3 V. The plateau around 4 V was observed for all Li/sample cells on first discharge. The maximum of initial discharge capacity was about 100 mAh/g down to 2.5 V for the Li/(50% Fe-substituted sample) cell, when the positive electrode was obtained by postannealing at 650degreesC in air. The capacity fading of the 4 V plateau could be suppressed by adjusting the Fe content to less than 50%, postannealing temperature between 600 and 700degreesC, and by 10% Ni substitution.