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The reaction of atomic bromine with several arylcyclopropanes has been investigated. For systems where the aryl moiety is phenyl or a-or yS-naphthyl, the expected SH2 pathway leading to cyclopropane ring cleavage is observed. In the case of 9-cyclopropylanthracene, however, an unprecedented hydrogen atom abstraction reaction is observed. These observations are explicable on stereoelectronic grounds. For the 9-anthryl system, the lowest energy perpendicular conformation finds the -C-H bond aligned with the -system, and consequently hydrogen abstraction is facile. In this conformation, the a-C-C bonds are deactivated toward bromine atom attack. For phenyl-and /3-naphthylcyclopropane, the bisected conformation is preferred and ring opening predominates. The ground-state conformation of -cyclopropylnaphthalene is unique in that it is midway between bisected and perpendicular. Although cyclopropane ring opening is the only detected process, quantitative data are presented which demonstrate that the rate of this process is diminished because of unfavorable stereoelectronic factors.Analysis of this reaction system with the Curtin-Hammett principle leads to a general statement of the requirements for a-hydrogen abstraction from cyclopropylarenes by bromine atom.