<p> A detailed study of the photoaddition of 3-phenylcyclohex- 2-enone and 2,3-dimethylbut-2-ene was undertaken in order to gain mechanistic information of the photochemistry of this enone. Photoadditions of this enone with cyclopentene, norbornene, and but-2-ene were also studied. Sensitization experiments showed that photoaddition with 2,3-dimethylbut-2-ene occurs when triplet sensitizers such as Michler's ketone or 2-acetonaphthone are used. The sensitized reactions are less efficient than the direct irradiation, except at infinite 2,3-dimethylbut-2-ene concentration, when the quantum yields for direct and sensitized processes are the same. This quantitative result may be explained by (a) inefficiency in the energy-transfer process from the sensitizer to enone 0 or (b) two excited states of enone being involved in the direct and sensitized processeso There are evidences that the excited state involved in the direct irradiation could possibly be the second triplet of the enoneo The presence of a higher enone tTiplet (T2) was established by the observed dimerization of norbornene. Cis-trans isomerization of but-2-ene was observed during photoaddition, which could also be attributed to a higher triplet, or to a 1,4-diradical intermediates Quenching studies provided an estimate of the lifetime of the reactive excited state in the direct irradiation. The energies of the electronic states of the enone were estimated from spectroscopic and chemical measurements. The formation of an enoneoolefin 1,4-diradical has been proposed to explain the effect of olefin concentration on the photoaddition. </p> / Thesis / Doctor of Philosophy (PhD)