The molecular complexes of bicycle [2. 2. 2] octa-2,5-diene with chlorine and bromine have been investigated using AMI semiempirical method. The geometric parameters and stabilisation energies related to the complexes have been calculated. The exo and endo adducts are found to be nearly isoenergetic. The greater stearic repulsions in the exo complex are apparently compensated by increased electronic stabilisation. The cationic intermediates of the electrophilic addition of chlorine and bromine to bicycle [2. 2. 2] octa-2,5-diene have been investigated by MNDO and ab initio methods. The results obtained reveal that the bridged endo-halogenium cation is relatively more stable than the bridged exo-halogenium cation for both chlorine and bromine. The rearranged cations have been found to be the most stable ones among the cationic intermediates formed in the reaction. It is plausible that the ionic addition reactions proceed via the most stable cations. Essentially, the rearranged addition products are predicted to form in the ionic addition reactions of chlorine and bromine To bicycle [2. 2. 2] octa-2,5-diene.