Stoichiometric LiNiO2 was delithiated using NOPF6 oxidation or acid leaching. We obtained a highly delithiated compound of Li0.1NiO2 by the latter process, whose electrochemical properties were substantially the same as those of LixNiO2 obtained by electrochemical delithiation. The chemically delithiated LixNiO2 sample allowed us to investigate the intrinsic thermal behavior of LixNiO2, as it was free from any influence of conductive agents or binders. The thermal decomposition mechanism of LixNiO2 is discussed based on the results of X-ray diffraction and thermogravimetric analysis.