AbstractThe photochemical behaviour of 4‐phenyl‐1,2‐dihydronaphthalene (8) is similar to that of unsubstituted 1,2‐dihydronaphthalene (1). The main product is the 1‐phenylbenzobicyclo[3.1.0]hex‐2‐ene (10). 3‐Phenyl‐ and 1‐phenyl‐1,2‐dihydronaphthalenes (14and17) behave differently.14is photostable and17yields only theexo‐isomer of 4‐phenylbenzobicyclo[3.1.0]‐hex‐2‐ene, instead of a mixture of theendo‐ andexo‐isomer as would be expected. The different results are probably due to differences in the steric requirements of the first inter