We investigated effects of delithiation on crystal, electronic structure, thermodynamic stability and physical property of LixNi0.5Mn0.5O2 by using a chemical delithiation treatment with ammonium per-oxodisulfate, and compared the results with those of electrochemical delithiation. X-ray diffraction and thermodynamic data indicated that there was no phase transition in the samples after the chemical delithiation treatment within the lithium range of 0.50 <= x <= 1.00. From the ICP measurements and potentiometric titration, it was found that the chemical treatment successfully remove Li from the structure without any elution of Ni and Mn, but it accompanied O-2 removal at an initial process of the delithiation, i.e. 0.80 <= x <= 1.00. Crystal and electronic structure analyses also suggested such a O-2 removal at the lithium content range in the chemical delithiation method, although the estimated structure changes below x = 0.80 showed similar tendencies to those by the electrochemical delithiation. (C) 2011 Elsevier B.V. All rights reserved.