amount of 1-(dihalomethylene)spiropentanes. The efficiency of the thermalrearrangement from the cyclopropylidenecyclopropanes to the 1-(dihalomethylene)spiropentane derivatives depended on the substituents and the reaction temperature. Reaction of diarylvinylidenecyclopropanes with diphenylcarbene and phenylthiocarbene gave the corresponding spiropentane derivatives. This type of thermal rearrangement