Abstract The synthesis of benzo [b] phenanthro [1, 2-d] thiphene (1), benzo [b] phenanthro [4, 3-d] thiophene (2), benzo-[b] phenanthro [2, 1-d] thiophene (3) and benzo [b] phenanthro [3, 4-d] thiophene (4) from appropriately substituted olefines by photochemical cyclodehydrogenation is described. The photolysis of olefin 9 gave a mixture of 4 and anthra [1, 2-b] benzo [d] thiophene (5).