We report the thermal behavior of Li1-yNiO2 up to 300 degrees C which we investigated using thermogravimetry (TG) and differential scanning calorimetry (DSC). The decomposition mechanism was studied using the X-ray diffraction data obtained for the thermally decomposed products. At about 200 degrees C Li1-yNiO2 turned into Li(1-y/(2-y))Ni1/(2-y)O which has a rock-salt structure. This was accompanied by oxygen evolution. A significant exothermic process was observed for compounds with y greater than or equal to 0.7 and this resulted in products with disordered rock-salt structures. Using these results, we propose a mechanism in which the exothermic behavior is caused by random cation mixing.