The gas-phase photodecarbonylation and photofragmentation reactions of substituted bicyclo [3.1. 0] hexan-3-ones have been studied in detail. Photolysis of these ketones yields 1, 3-dienes, vinylcyclopropanes, and 1, l-dienes as detectable products. The possible mechanisms for these reactions are discussed in light of the regiochemical and stereochemical results obtained. In addition, methyl substitution at C-6 and C-2 of these ...