AbstractIrradiation of 1‐(9‐phenanthryl)‐4‐phenylbutenyne (1) in aprotic solvents containing oxygen gives two products, namely a photocyclization product, 1‐phenyltriphenylene3, as is usually formed from diarylbutenynes, and a photooxidation product (2), derived from cyclopropapyrone. The formation of the latter product is rather specific to the butenyne1.Both products arise from singlet‐ excited butenyne, probably via the same cycloallene intermediate4. Supposedly, this intermediate is converted by triplet oxygen into a 1,2‐dioxetane, having an exocyclic double bond, from which the oxidation product can be formed by ring opening and further rearrangements. The negligible photooxidation of other butenynes is ascribed to the much shorter lifetime of the corresponding c