Acetone sensitized photolysis of 2-phenyl- and 2-methyl-2-phenylcyclohepta-3,5-dienones gives in about equal yield 2-substituted bicyclo[3.2.0]hept-6-en-3-ones and 6-substituted cyclohepta-2,4-dienones as the major products, while on direct irradiation both dienones undergo mainly decarbonylation to produce unstable trienes.