Cyclopropane has been prepared with three deuterium atoms on the same face (6, cyclopropane-cis-1,2,3-d3) in order to determine the mechanism of electrophilic halogenation. This labeling pattern permits determination of the stereochemistry of both electrophilic and nucleophilic attack. Bromination and chlorination proceed under polar conditions to give 1,3-dihalopropane as the predominant product. The temperature dependence of the vicinal H-H coupling constant in the unlabeled dihalopropanes confirms that the major conformer has gauche arrangements over both C-C bonds (the GG conformer). The products were stable to rearrangement under reaction conditions. The magnitude and temperature dependence of the vicinal H-H coupling constant of the labeled 1,3-dibromo- and 1,3-dichloropropanes, observed under conditions of deuterium decoupling, were uniquely consistent with a retention-inversion stereochemistry. The observed stereochemistry is consistent with initial edge halogenation with retention to form a four-membered bromonium ion, followed by nucleophilic ring opening with inversion. This initial intermediate is the most stable according to ab initio calculations.