Summary The reversible isomerization of methylcyclopentane to cyclohexane in the presence of alumi-num bromide-hydrogen bromide catalyst has been studied using high vacuum technique. It was found that under certain controlled conditions methylcyclopentane does not undergo isomerization to cyclohexane unless cyclohexene or cyclohexyl bromide in. amounts of about 0.05 mole per cent. or higher are present. The effect of olefins, ...