The molecular complexes formed by the treatment of bromine with benzonorbornodiene have been studied by the AM1 semiempirical method and their stable configurations and also stabilization energies have been determined. It has been found that the molecular complexes formed by the attack of bromine on the double bond of benzonorbornadiene, from the exo direction are more stable than those formed from the endo direction. Possible various cations and radical intermediates of the electrophilic addition reaction of bromine to benzonobornadiene have been investigated by MNDO and ab initio methods. The nonclassical delocalized bromocarbonium cation is the most stable among these cations according to both methods, and the ionic addition reaction occurs via this cation. (C) 1999 Elsevier Science B.V. All rights reserved.