The chlorine-benzonorbornadiene molecular complexes have been investigated by AM1 semiempirical method and their stable configurations have been determined. The geometric parameters and stabilisation energies of the complexes have been calculated. Exo-molecular complex has been found to be relatively more stable than the endo-complex. The various cationic and radicalic intermediates of the electrophilic addition of chlorine to benzonorbornadiene have been studied using MNDO and ab initio methods. It has been found that the exo-chloronium cation is 7.9 kJ/mol and 30.6 kJ/mol relatively more stable than the endo-chloronium ion according to MNDO and STO-3G methods, respectively. The non-classical delocalized chloronium cation has been found to be the most stable one among the cationic intermediates formed in the reaction, and so the ionic addition reaction proceeds via this cation. According to the methods used in the study, the exo-chloro radical is relatively more stable than the endo-chloro radical. Radicalic addition is predicted to occur via these intermediate radicals.