The influence of 5-substituents on the course of bromination of N-alkoxycarbonyl-5, 6-dehydroisoquinuclidines la-f has been investigated. Methyl, chloro, phenyl, and acetoxy substituents, which stabilize adjacent cations, result in products which do not involve skeletal rearrangement. With hydrogen or form1 substitution at C5 bromination is accompanied by nitrogen participation, which results in skeletally rearranged products. The bromination of 5-formyl olefin 1f to give 2-endo bromo stereoisomer 18 is consistent with the formation of an unusual four-membered ring bromonium ion.