The thermal decomposition of NOClO4 (powder, or fragments of pressed pellets) and of NO2ClO4 (granules) has been studied gasometrically and by X-ray diffraction. During the decomposition of NOClO4, NO2ClO4 is formed as an intermediate. The following mechanism is suggested for the decomposition of NOClO4: NOCl4→ClO3+NO2, 3NOClO4+ClO3→ 1 2+3NO2ClO4, 3NO2ClO4→3ClO2+3NO2+3o2. NO2ClO4 is partly reduced to NOClO4. Independently of the initial sample, the reaction mixture will contain both perchlorates.