The activation parameters of the thermal rearrangements of the trans-divinylcyclopropanes 1 and 2, and of cis-divinylcyclopropane 3 to give cycloheptadiene 5 have been determined. In addition it has been shown that trans-divinylcyclopropane-cycloheptadiene rearrangements also occur under irradiation by a mercury lamp in benzene at 40°. These low-temperature in vitro rearrangements suggest that the biosynthesis of the cycloheptadienes in Dictyopteris (dictyopterenes C′ and D′) may proceed by the same reaction(s).