The mercury (3P1) photosensitized internal cycloaddition reactions of 1,4-pentadiene, 2-methyl-1,4- pentadiene, 2,4-dimethyl-1,4-pentadiene, 1,5-hexadiene, 2-methyl-1,5-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.2.0]alkane and a crossed adduct of the formula bicyclo[n.1.1]alkane. In all of the 1,4-pentadienes the former predominated, in the 1,5-hexadienes the latter predominated, while in 1,6-heptadiene the former once again predominated. A mechanism is proposed to explain this. With an increase in the complexity of the molecule the isomerization to give cyclopropyl derivatives decreased in importance, and internal cycloaddition accounted for as much as 60% of the diene. A dimerization reaction is proposed to account for the remaining diene that reacted. © 1967, American Chemical Society. All rights reserved.