Metastable transitions have been observed in the mass spectra of benzene, hexadeutero-benzene, 1,4-dideuterobenzene and 1,3,5-trideuterobenzene using a double-focusing mass spectrometer. Decomposition maps have been obtained for each substance and decomposition reactions leading to the production of all significant fragment ions have been suggested. Metastable transitions of several doubly-charged species were found. Loss if identity by the H and D atoms of the partly-deuterated benzenes was observed in metastable transitions in their mass spectra, indicating that rapid isomerization occurs before fragmentation.