Thermolysis of 2,2,6,7‐Tetramethyl‐1,7‐epidioxybicyclo[4.3.0]non‐8‐ene.Dye‐sensitized photooxygenation of the cyclopentadiene derivative1gives three products: the peroxide2aand the monocyclic ketones5and8. 5and8are probably formed by rearrangement of the unstable peroxide2b. Thermolysis of2agives, besides5and8, the furylketone9and the cyclooctenone derivative10. The structures of the new compounds have been established by means of spectroscopic methods and chemical transformations. A reaction mechanism for the formation of5, 8, 9and10from2ais proposed. Irradiation of10leads to the two diastereoisomerictranscyclooctenon