Thermal behavior of Li1−x NiO2 ( R 3 ̄ m or C2/m) up to 400°C was studied using thermogravimetry (TG), differential scanning calorimetry (DSC), and high-temperature X-ray diffraction measurements and its decomposition mechanism was proposed. Delithiated Li1−x NiO2 was thermally decomposed to LiNi2O4 spinel (Fd3m) at the temperature range of 180–250°C. For x≤0.5, Li1−x NiO2 was decomposed to LiNiO2 and LiNi2O4 and the fraction of the spinel in the decomposed product increased almost linearly with x. For 0.5<x≤0.8, Li1−x NiO2 was converted into LiNi2O4 spinel and this reaction was accompanied by oxygen evolution. Li1−x NiO2 of all compositions turned into a rock-salt phase (Fm3m) with oxygen liberation at temperatures above 270°C. The temperature for the decomposition of Li1−x NiO2 to a spinel or a rock-salt phase decreased with x in Li1−x NiO2. The thermal behavior of Li1−x NiO2 could be interpreted as an overlap of the exothermic rearrangement of cations (nickel and lithium ions) to form a spinel or a rock-salt phase and the endothermic oxygen evolution reaction.