Layered NaNi0.5Mn0.5O2 (space group: R (3) over barm), having an O3-type (alpha-NaFeO2 type) structure according to the Delmas' notation, is prepared by a solid-state method. The electrochemical reactivity of NaNi0.5Mn0.5O2 is examined in an aprotic sodium cell at room temperature. The NaNi0.5Mn0.5O2 electrodes can deliver ca. 105-125 mAh g(-1) at rates of 240-4.8 mA g(-1) in the voltage range of 2.2-3.8 V and show 75% of the initial reversible capacity after SO charge/discharge cycling tests. In the voltage range of 2.2-4.5 V, a higher reversible capacity of 185 mAh g(-1) is achieved; however, its reversibility is insufficient because of the significant expansion of interslab space by charging to 4.5 V versus sodium. The reversbility is improved by adding fluoroethylene carbonate into the electrolyte solution. The structural transition mechanism of Na1-xNi0.5Mn0.5O2 is also examined by an ex situ X-ray diffraction method combined with X-ray absorption spectroscopy (XAS). The staking sequence of the [Ni0.5Mn0.5]O-2 slabs changes progressively as sodium ions are extracted from the crystal lattice. It is observed that the original O3 phase transforms into the O'3, P3, P'3, and P3 '' phases during sodium extraction. XAS measurement proves that NaNi0.5Mn0.5O2 consists of divalent nickel and tetravalent manganese ions. As sodium ions are extracted from the oxide to form Na1-xNi0.5Mn0.5O2., nickel ions are oxidized to the trivalent state, while the manganese ions are electrochemically inactive as the tetravalent state.