SCF-MO-LCAO AM I calculations have been performed for 1:1 molecular system of bromine with homobenzonorbornadiene (HBNB) and their stable configurations have been determined. The stable configurations of the HBNB-Br-2 system correspond to HBNB-Br-2 (exo) and HBNB-Br-2 (endo) molecular complexes which are formed by the exo and endo orientations of Br-2 molecule to the double bond of HBNB in axial position, respectively. Exo-molecular complex has been found to be relatively more stable than the endo-complex. The cationic intermediates of the reaction have been studied by ab initio SCF method in STO-3G and STO-3G* basis and also by MNDO semiempirical route. Exo-bromonium cation is found to be more stable than endo-bromonium cation according to both the methods. The nonclassical delocalized bromocarbonium cation is the most stable among these cations according to both methods, and the ionic addition reactions occur via this cation.