Abstract On ultraviolet irradiation in toluene, cyclopent-2-en-one (1a) and 3-methylcyclopent- 2-en-one (1b) undergo dimerisation ([RIGHTWARDS ARROW] 3a, b, 4b), whereas 3-t-butyl- cyclopent-2-en-one (1e) and the bicyclo [3.2. 1] octenone 2a only react with the solvent ([RIGHTWARDS ARROW] 5e–7e, 8–10). The 3-ethyl-(1c) and 3-isopropyl-cyclopent-2-en- ones (1d) afford by dimerisation products 3c, d, 4c and by reaction with the solvent 5c, d, ...