Irradiation of the AlEtCl2 complex of endo-tricyclo[5.2.1.02,6]deca-4,8-dien-3-one (1AL) yielded the AlEtCl2 complex of exo-tricyclo[5.3.0.02,6]deca-4,8-dien-3-one (3AL). The structure of 3, which was obtained on treatment of the irradiated solution with water, was established by its reduction to give the known exo-tricyclo[5.3.0.02,6]decan-3-one. The photoisomerization of 1AL could not be driven to completion as 3AL underwent a photoisomerization to regenerate 1AL. Control experiments showed that the interconversions of 1AL and 3AL were photochemically and not thermally induced. Irradiation of AlEtCl2 complex of trideutero derivative 6 did not lead to any deuterium scrambling in the starting material, indicating that no hidden skeletal isomerization such as a 2-carbon plus 3-carbon cycloaddition is taking place during this isomerization. The mechanisms of the photoisomerizations are discussed.