AbstractAcetal substitution reactions of α‐halogenated five‐ and six‐membered rings can be highly stereoselective. Erosion of stereoselectivity occurs as nucleophilicity increases, which is consistent with additions to a halogen‐stabilized oxocarbenium ion, not a three‐membered‐ring halonium ion. Computational investigations confirmed that the open‐form oxocarbenium ions are the reactive intermediates involved. Kinetic studies suggest that hyperconjugative effects and through‐space electrostatic interactions can both contribute to the stabilization of halogen‐substituted oxocarbenium ions.