Electron impact fragmentation of 18 compounds from the series of 9-sila-9,10-dihydro-3-azaanthracene has been studied. The Formation of [M RH]+ ions where R is a substituent at the silicon atom is the main fragmentation route for siladihydroazaanthracenes. C(10)-Oxo derivatives lead to [M R′]+ (maximum intensity ion) and [M R′ RH] ions. These regularities may be used for the identification and structural verification of the above compounds. A specific feature of the mass spectra of siladihydroazaanthracenes is the randomisation of the hydrogen atoms at the C(10) position with the hydrogen atoms of the pyridine and benzene rings of the siladihydroazaanthracene ring. © 1980.