Structural changes for a series of nickel based layer structured cathode materials during heating from 25 to 450°C in the presence of electrolyte were studied using time-resolved X-ray diffraction (XRD). These samples were electrochemically delithiated to Li0.33NiO2, Li0.27Ni0.8Co0.15Al0.05O2, and Li0.2Ni0.5Mn0.5O2. The structural changes of all these three samples follow a same trend, first convert from a layered structure to a disordered spinel structure, then the disordered spinel phase transforms to the NiO-like phase as the temperature increases. However, the onset temperature of the thermal decomposition reactions strongly depends on the nickel content of the material. The lower the nickel content, the higher the decomposition temperature. At the end of heating at 450°C, the metallic nickel structure was observed only for Li0.33NiO2 sample.