Abstract: The mercury (3P1> photosensitized internal cycloaddition reactions of 1, 6pentadiene, 2-methyl-l, 4-pentadiene, 2, 4-dimethyl-i, Cpentadiene, 1, 5-hexadiene, 2- methyl-l, 5-hexadiene, 1, 5-cyclooctadiene, and 1, 6-heptadiene have been studied at 1-atm pressure at the boiling points of the respective compounds. With the exception of 1, 5- cyclooctadiene, all the compounds gave a parallel adduct of the general formula bicyclo [n ...