The photochemical 1,3-addition reaction of benzene, several alkyl benzenes, and anisole to a variety of linear, monocyclic, and bicyclic olefins has been studied from both mechanistic and synthetic points of view. The addition takes place with retention of the stereochemistry at the olefin and with a high degree of orientational specificity. The reaction is believed to originate in the lowest singlet excited state of the aromatic and may involve an exciplex as an intermediate. The quantum yields for the addition at the limit of the olefin concentration fall in a range from 0.1 to 0.5. A model for the exciplex which accounts for all of the observations and has considerable predictive value for the reactivity and orientational effects that are observed in different olefins has been proposed. Two classes of reactions of the photochemical adducts have been investigated in detail. These are the thermal rearrangements, particularly of the adducts of benzene to cyclobutene, and the acid catalyzed rearrangements. All of the 1,3-adducts are sensitive to pyrolysis. At about 280 degrees, in the vapor phase in a static system, the reactions that are observed are reversal of the photoaddition, a 1,5-sigmatropic shift of hydrogen, and the vinylcyclopropane rearrangement. The adducts of cyclobutene also undergo a (2'-vinylcyclopropyl) cyclobutane rearrangement, a hitherto unknown 1,5-sigmatropic shift of a C-C bond. The course of this reaction has been studied by deuterium labeling. Acid catalyzed opening of the 1,3-adducts results in the breaking of the cyclopropane ring to give bicyclo 3.2.1 octane-8-01 derivatives. The sterochemistry of this rearrangement was established.