The methylene blue (MB)-sensitized photo-oxygenation of indene (1) in CH3OH gave trans-2-hydroperoxy-1-methoxyindane (9), homophthalaldehyde (4), and its cyclic acetal 8. The same reaction conducted in acetaldehyde gave only 4. Repetition of the photo-oxygenation of 1 in slightly acidic aq. acetone (1:9) gave essentially trans-2-hydroperoxy-1-hydroxyindane (14) together with some cis-5,6-dihydro-3,3-dimethyl-1,2,4-trioxinol[1,2-e]indene. The MB-sensitized photo-oxygenation of 1,2-dihydronaphthalene (16) in CH3OH gave l,4-dihydro-1-hydroperoxynaphthalene (17) and trans-l,2,3,4-tetrahydro-2-hydroperoxy-1-methoxynaphthalene (19). In acetaldehyde, 16 furnished only 17. In aq. acetone (1:9) 16 gave 17 and trans-2-hydroperoxy-1-hydroxy-1,2,3,4-tetrahydronaphthalene (23). Product compositions were rationalized in terms of the 2-peroxides of the 1-cation of 1 and 16, namely 11 and 20. Treatment of 23 with aldehydes and ketones on catalysis with Amberlyst-15 or trimethylsilyl trifluoromethanesulfonate afforded the trans-fused 1,2,4-trioxanes 25. The 1,2-dioxetanes of 1 and 16 condensed with acetaldehyde on catalysis with CF3COOH to give the cis-fused 1,2,4-trioxanes 12 and 22.