The electronic and geometric structures of endo,endo-tetracyclo[220.127.116.11(3,6).0(2,7)] dodeca-4,9-diene(TTDD) molecule have been investigated by ab initio and DFT/B3LYP methods with the 6-31G* and 6-311G* basis sets. The double bonds of TTDD molecule have been endo-pyramidalized. The structure of pi-orbitals and their mutual interactions for TTDD molecule have been investigated. The cationic intermediates and products obtained as a result of the addition reaction have been studied using HF/6-311G*, HF/6-311G**, B3LYP/6-311G* and B3LYP/6-311++G**/IB3LYP/6-311G* methods. The bridged chloronium cation isomerises into the more stable N- and U-type cations and the difference between the stability of these cations is little. The direction of the addition reaction has been determined by the direction of the rearrangement of the bridged chloronium ion into the N- and U-type cations. The N- and U-type reaction products are obtained as a result of the reaction, which takes place via the cations in question. The stability of exo,exo and exo,endo isomers of N-type product are nearly the same and the formation of both isomers is feasible. The U-type product is basically formed from exo,exo-isomer. Although, the U-type cation is 3.037 kcal mol(-1) more stable than the N-type cation, the U-type product is 2.769 kcal/mol more unstable than the N-type product. The reaction products are controlled kinetically.