endo-Tricyclo[4.2.1.02,5]non-7-ene-3,4,9-trione (14) is prepared from the 5,5-diethoxy-cyclopcntadiene-diethyl maleate Diels-Alder adduct. Photolysis of the trione in CD2Cl2 solution at 192 K forms norborna-2,5-dien-7-one (2). The latter substance is thermally unstable and decomposes to benzene and CO with ΔG‡ =15 kcal mol-1. This appears to be the lowest activation energy for a cycloreversion yet measured. A correlation is found for the kinetic and thermodynamic stabilities in a series of 18 orbital symmetry allowed cycloreversions by plotting ΔG‡ vs ΔHr.