Full geometric optimization of endo-tricyclo[18.104.22.168(2,4)]oct-6-ene (endo-TCO) by ab initio and DFT methods allowed us to investigate the structure of the molecule. The double bond is endo-pyramidalized and its two faces are no longer found to be equivalent. The exo face of the double bond has regions with far more electron density (q(i,HOMO)) and more negative electrostatic potential. The endo-TCO-Br-2 system was investigated at the B3LYP/6-311+G** level and the endo-TCO center dot center dot center dot Br-2(exo) molecular complex was found to be relatively more stable than the endo-TCO center dot center dot center dot Br-2(endo) complex. The cationic intermediates of the reaction were studied by ab initio and DFT methods. The bridged exo-bromonium cation(I) is relatively more stable than the endo-bromonium cation(II). An absolute exo-facial selectivity should be observed in the addition reaction of Br-2 to endo-TCO, which is caused by steric and electronic factors. The nonclassical rearranged cation IV was found to be the most stable ion among the cationic intermediates and the ionic addition occurs via the formation of this cation. The mechanism of the addition reaction is also discussed.