The geometry and the electronic structure of tricyclo[126.96.36.199(2,5)]dodeca-1,5-diene (TCDD) molecule were investigated by DFT/B3LYP and /B3PW91 methods using the 6-311G(d,p) and 6-311++G(d,p) basis sets. The double bonds of TCDD molecule are syn-pyramidalized. The structure of p-orbitals and their mutual interactions for TCDD molecule were investigated. Potential energy surface (PES) of the TCDD-Br-2 system was studied by B3LYP/6-311++G(d,p) method and the configurations [molecular charge-transfer (CT) complex, transition states (TS1 and TS2), intermediate (INT) and product (P)] corresponding to the stationary points (minima or saddle points) were determined. Initially, a molecular CT-complex forms between Br-2 and TCDD. With a barrier of 22.336 kcal mol(-1) the CT-complex can be activated to an intermediate (INT) with energy 15.154 kcal mol(-1) higher than that of the CT-complex. The intermediate (INT) then transforms easily (barrier 5.442 kcal mol(-1)) into the final, N-type product. The total bromination is slightly exothermic. Accompanying the breaking of Br-Br bond, C1-Br, C5-Br and C2-C6 bonds are formed, and C1 = C2 and C5 = C6 double bonds transform into single bonds. The direction of the reaction is determined by the direction of intramolecular skeletal rearrangement that is realized by the formation of C2-C6 bond.