O-3-type layered structure NaNi0.5Mn0.5O2 was successfully synthesized by solid state reaction. NaNi0.5Mn0.5O2 was tested at room temperature using two-electrode Na/NaNi0.5Mn0.5O2 coin cells with NaClO4/Propylene Carbonate electrolyte. NaNi0.5Mn0.5O2 delivered a reversible capacity of 120 mAh/g during the first charge to 3.8 V, which corresponds to the de-intercalation of about 0.5 mol Na per mole transition metal. The reactivity of de-intercalated NaNi0.5Mn0.5O2 (similar to Na0.5Ni0.5Mn0.5O2) with ethylene carbonate/diethyl carbonate (EC:DEC) solvent and with NaPF6/EC: DEC electrolytewas studied using Accelerating Rate Calorimetry (ARC). X-ray diffraction was used to study the products of the reactions of Na0.5Ni0.5Mn0.5O2 with solvent or electrolyte after the ARC experiments. Na0.5Ni0.5Mn0.5O2 shows high reactivity in both solvent and electrolyte. In solvent, Na0.5Ni0.5Mn0.5O2 decomposes at elevated temperature to form nickel manganese oxide and Ni metal, and the released oxygen reacts with the solvent. In NaPF6-based electrolyte, the reaction mechanism changes completely as new reaction products, Na-3(Ni,Mn)F-6 and Na(Ni, Mn)F-3, are formed in a new low temperature exothermic reaction. These results imply important consequences regarding safety for those researchers selecting electrolytes and electrode materials for Na-ion batteries. In both cases, NaF was found as a product of Na0.5Ni0.5Mn0.5O2 reacting with PVDF binder. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.060207jes] All rights reserved.