The dehydrogenation of 1∶2∶3∶4-tetrahydrodibenzothiophene (I) was studied by using various dehydrogenating agents in order to prepare dibenzothiophene (II) in optimum yield under mild conditions. Dehydrogenation by means of selenium or palladised carbon gave good yields of (II), but the reaction was carried out at high temperatures (>275°). By employing N-bromosuccinimide, the dehydrogenation could be carried out in a facile manner at lower temperatures (76°). These results are of value in the synthesis of derivatives of (II). Thus, whereas dehydrogenation of the chlorotetrahydrobenzothiafluorene (III) by treatment with selenium gave the benzothiafluorene (IV) by simultaneous dehydrogenation and dechlorination, dehydrogenation with N-bromosuccinimide gave the chlorine-containing benzothiafluorene (V).