Abstract Irradiation of 2,5-diphenyloxadiazole 1 with benzothiophene 6a gives 3-benzoylbenzothiophene 7, its benzoylhydrazone 8, and/or the oxadiazepine 9; the yields depended on the nature of solvents. With benzophenone as a sensitizer, the photochemical reaction of 1 with 6a forms the [2+2] cycloadduct 12. It is found that 9 is photochemically dissociated to 1 and 6a. In the case of 2-methylbenzothiophene 6b, 3-benzoyl-2-methylbenzothiophene benzoylhydrazone 18 is formed, and with benzophenone as a sensitizer the [2+2] cycloadduct 19 is obtained. In the absence or presence of benzophenone, however, irradiation of 1 with 3-methylbenzothiophene 6c gives the [2+2] cycloadduct 20. The photochemical reaction of 1 with 6a or 6b in the presence of iodine gives the corresponding 3-benzoylbenzothiophene, 7 or 21, and benzoylhydrazone, 8 or 18, respectively. In the case of 6c, however, the [2+2] cycloadduct 23 is formed, together with 2-benzoyl-3-methylbenzothiophene 22. Mechanistic considerations of these reactions are also described.