Thermal behavior of electrochemically delithiated Li1-xNiO2 (R (3) over barm or C2/m) up to 400 degreesC was studied by thermogravimetry. differential scanning calorimetry, high temperature X-ray diffraction, and X-ray absorption measurements, and its thermal decomposition mechanism was proposed. Delithiated Li1-xNiO2 was thermally decomposed to a spinel phase (Fd3m) at around 220 degreesC. For x less than or equal to 0.5, Li1-xNiO2 was decomposed to LiNiO2 and LiNi2O4 and the fraction of the spinel in the decomposed product increased almost linearly with x. For x > 0.5, Li1-xNiO2 was converted into the spinel and this reaction was accompanied by oxygen evolution. On further heating, delithiated Li1-xNiO2 of all compositions turned into a rock salt phase (Fm3m) with NiO structure. The temperature for the decomposition of Li1-xNiO2 to a spinel was independent of x, however, the temperature for the decomposition to a rock salt phase decreased with increasing x. The thermal behavior of Li1-xNiO2 could be interpreted as an overlap of the exothermic rearrangement of nickel and lithium cations to form a spinel or a rock salt phase and the endothermic oxygen evolution reaction.