Abstract: The experimental data show that the EImass spectra of 3,6-dinitro-dibenzopyriodonium halides (X = Cl, Br, I) and 3,6-dinitro-9-one-diben-zopyrdonium halides (X = Cl, Br, I) are the mixed mass spectra of their thermolysis products produced inside the instrument. The adsorbed solvent, formic acid, in the samples (Ⅳ-Ⅵ)takes part the thermolysis process. By the electron collision induced fragmentation, one can easily identify 2-halo-2' -iodo-4, 4' -dinitro-dibenzoketone, the main thermolysis product of 3,6-dinitro-9-one-dibenzopyriodonium halides (X = Cl, Br, I).