AbstractElectron impact mass spectra were measured for five isomers of pyridinobenzanthrones and three isomers of benzobenzanthrones. The fragmentation pattern and intensity of M2+, [M – H]+, [M – CO]i+, [M – CO – H(or 2H)]i+ and [M – CO – HCN]i+ (i = 1, 2) ions indicated remarkable differences and very interesting features according to the isomers with or without nitrogen and condensation positions of a pyridino or benzo ring to the benzanthrone skeleton. Further, the competition of decompositions through [M – H]+, [M – CO]+˙ or [M – HCN]+˙ ions was confirmed by the observation of metastable ions and the appearance energies of fragment ions. Interesting observations from these results were expulsion of an H atom in close proximity to the area around an OC group, a weak bonding interaction between sp2 CH and an OC group, inducing specific hydrogen rearrangement, and characteristic charge localization on heteroatoms.