The electronic and geometric structures of homobenzonorbornadiene (HBNB) molecule were investigated by ab initio HF/3-21G*, HF/3-21G**,HF/6-31G* and HF/6-311G* methods. The double bond of molecule is endo pyramidalized. HF/3-21G* calculations have been performed for 1 : 1 molecular system of chlorine with HBNB and their stable configurations have been determined. The stable configurations of the HBNB-Cl-2 system correspond to HBNB (...) Cl-2(exo) and HBNB (...) Cl-2(endo) molecular complexes which are formed by the exo and endo orientation of Cl-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 3-21G* 3-21G**, 6-31G* and 6-311G* basis. Exo-chloronium cation is found to be more stable than endo-chloronium cation. Exo-facial selectivity should be observed in the addition reaction to HBNB of chlorine. Nonclassical delocalized (IV) cation is more stable than the classical rearranged (V) cation. For this reason, the conversion of the cation(IV) to ionV is not easy. The nonclassical delocalized chlorocarbonium cation(IV) is the most stable ion among the cationic intermediates, and the ionic addition reactions occurs via this cation. The mechanism of the addition reaction also discussed. (C) 2004 Elsevier B.V. All rights reserved.