Compounds of the 3,4‐dihydro‐ionone series as models for the photochemistry of γ, δ‐ and δ,ϵ‐ unsaturated ketones and aldehydes.The photochemistry of γ, δ‐ and δ,ϵ‐unsaturated carbonyl compounds of the dihydro‐ionone series has been studied, with special attention to the investigation of oxetane formationversushydrogen abstraction.UV.‐irradiation of the dihydro‐β‐ionone compounds with structureA(1,7,14,18,24,29) led to isomeric ethers with structuresB(2,8,15,19,25,30),C(3,9,16,20,26,31) andD(4,21,27), isomeric bicyclic alcohols with structureE(5,10,17,22,28), and photoreduction products with structureF(6,11,12,13). Photolysis of dihydro‐γ‐ionone (32) gave a complex mixture containing fragmentation product35, hydrocarbon36, β‐ambrinol (34), oxetane33, as well as dihydro‐β‐ionone (1) and three of its photoproducts (2,3,5). The dihydro‐α‐ionone compounds37and40gave mixtures of fragmentation products and the oxetanes38and41. Irradiation of the side‐chain homologues42and45yielded43, which photo‐cyclizes to44. In contrast,3,4‐dihydro‐3′,4′‐dehydro‐β‐ionone (46) gave merely the isomeric open‐chain triene‐ketone47.The structures assigned to the ethers2,3,33,38and to the alcohols5,10,13could be confirmed by chemical reactions and mutual interconversions. The structure of the ether21had to be established by X‐ray analysis, details of which are described.A novel intramolecular hydrogen transfer is involved in formation of ethersB. The photocyclizationA → Dprobably proceeds by addition of the carbonyl‐C atom to the double bond (A → h), followed by methyl (1 → 2)‐shift