By acid catalysis, gem-dihalogenocyclopropylcarbinols are converted into homoallylic α-dihalogenated cations which in general eliminate the hydrogen halide and lead to the 3-halogenopentadienyl cations from which cyclopentenones can be formed by cyclisation. For bicyclo[4.1.0]heptanes, aromatisation of the 3-halogenopentadienyl cation in benzyl bromide is observed.