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Direct and sensitized photolyses of 4,5-dimethylcyclohexene vapor were investigated using a stereochemical approach at room temperature over the pressure range from 1 to 9 Torr. In the direct photolysis, decomposition of the retro-Diels-Alder type was the main course of the reaction, leading to the formation of 2-butene and 1,3-butadiene with the retention of the original configuration; 3,4-dimethyl-1-methylenecyclopentane was also formed as a minor product from a pressure-independent process. Mercury sensitization, on the other hand, produced cis- and trans-2-butenes, butadiene, 2,3-dimethyl-1-vinylcyclobutane, and cis-transisomers of the substrate together with radical-induced products, i.e., hydrogen, 1,2-dimethylcyclohexane, and dimeric products. The intermediacy of a “hot,” “triplet” biradicalwas suggested from the pressure-dependence of the trans- to cis-2-butene ratio in the mercury sensitization. The differences in the results between the direct and sensitized photolyses are interpreted in terms of spin correlation effects.