The decomposition of mixtures of metal oxides with NOClO4 or NO2ClO4 was studied by gasometry, thermogravimetry, D.T.A. and i.r. absorption. It is shown that NOClO4 is first converted to NO2ClO4, which reacts with some of the oxides to form the corresponding metal perchlorate. The weight loss indicated by the thermogravimetric curves is in quantitative accord with this description and the mechanism previously proposed for the decomposition of NOClO4.