The photoreactivity of 1,3-dihydro-2H̠ -benzimidazole-2-thione ( 1a ) and 2(3H )-benzothiazolethione ( 1b ) has been studied in several solvents in the presence of oxygen with or without methylene blue as a sensitizer. The corresponding disulfides (5a, b), 2-oxo derivatives (2 a , b), benzimidazole and benzothiazole (11a, b), and elem ental sulfur have been characterized as the main products. The ... [Show full abstract] UV irradiation of the disulfides 5a , b results in the formation of the same product pattern as the thiones 1 a , b. Photolysis of 2-chlorobenzothiazole (12) in acetonitrile gives a mixture of 2 b , 2,2'-dibenzothiazole (13). while photolysis of 12 in ethanol yields 2b and 11 b . Products and mechanisms are discussed and compared with previous studies in this area.