Photochemical rearrangement of 2,2,5,5-tetramethyl-1,3-cyclohexanedione (I) yields predominantly and enol δ-lactone V with an exocylic olefinic bond. A small amount of its more stable isomer VIII is also produced. Similar treatment of 2,2-dimethyl-1,3-cyclohexanedione (II) exclusively affords an exocylic enol δ-lactone VI. These enol lactones V and VI are reconverted photochemically into the respective diketones I and II in low yields, while VIII gives fragmentation products. Photolysis of an open-chain diketone, 3,3-dimethyl-2,4-pentanedione (XVI), gives the isomeric enol ester XXI along with biacetyl and 1,4-diketone XX, whereas photolysis of 2-acetyl-2-methylcyclohexanone (XXII) involves the so-called Type II reaction leading to a ring-cleavage product XXVI. The mechanistic implications of the photochemistry of I, V and VIII have been discussed.