Dihydrodiols are derived from intermediary arene oxides during the metabolism of polycyclic aromatic hydrocarbons. In this study, the mass spectra of the trimethylsilyl ether derivatives obtained from the 5,6‐dihydrodiol and 8,9‐dihydrodiol of 7,12‐dimethylbenz[a]anthracene were compared. The mass spectral fragmentation patterns indicate some key features in distinguishing a K‐region dihydrodiol from its non K‐region isomer. The 8,9‐dihydrodiol, a non K‐region metabolite, exhibits a greater tendency to undergo a ringcontraction fragmentation process while the K‐region 5,6‐dihydrodiol participates preferably in other fragmentation pathways. The data presented in this report will be useful in characterizing K‐region and non K‐region metabolic epoxidation of polycyclic aromatic hydrocarbons. Copyright © 1979 Heyden & Son Ltd