The mass spectra of 15 compounds of the 4-azaphenanthrene series and their deutero analogs with bulky R substituents (R=-C≡CPh, -CH=CHPh, CHBr-CHBrPh,-CH=CHC6H4OCH3-p, -COOH, -COOC2H5, and -CONHNH2) in the ortho position relative to the nitrogen atom were studied. An intense [M-H]+ ion peak, the appearance of which is evidently due to a process involving cyclization of the carbon atom of the side chain with the nitrogen atom, is formed in the fragmentation of the styryl derivatives. The fragmentation of the compounds with a methyl group in the ortho position relative to the styryl grouping is accompanied by the formation of an intense [M-Ph]+ ion peak. An "ortho effect" of the carboxyl and carbethoxy groups adjacent to the methyl substituent is observed in the mass spectra of the investigated compounds. The data obtained make it possible to establish the structures of bulky substituents in the ortho position relative to the ring nitrogen atom and the position of the methyl group in the pyridine ring of monomethylated azaphenahthrenes. © 1979 Plenum Publishing Corporation.