Photolysis of the dibenzobarrelene ester lactone, C21H16O4 (1), can produce several products, the structure of one of which, 8b-methyl 4b-(1-hydroxyethyl)-4b,8b,8c,8d-tetrahydrodibenzo[a,f]cyclopropa[cd]pent alene -8b,8c-dicarboxylate carbolactone (2), has been determined by X-ray methods. The molecule of (2) contains a three-membered ring and has geometry and dimensions similar to those of related materials.