AbstractThe formation of the bicyclo[3.3.0]octane ring system by isomerization of perhydronaphthalene and perhydroindane has been studied. Experiments were performed in the liquid phase under the influence of aluminium halides and in the gas phase at 300‐500° in the presence of a silica‐alumina catalyst. In both types of reaction a great number of compounds were formed.From the liquid‐phase reaction product of perhydronaphthalene a hydrocarbon carrying two methyl groups per molecule could be separated by preparative GLC; its structure could not be determined.In the liquid as well as in the gas‐phase experiments with perhydroindane the five isomeric methyl‐cis‐bicyclo[3.3.0]octanes were formed besides a large number of unidentified degradation products.Experiments with the individual isomers revealed that under the conditions of the gas‐phase reactions their stabilities differ considerably; 1‐methyl‐cis‐bicyclo[3.3.0]octane is the most stable, the endo‐2‐methyl derivative