Potential energy surface (PES) of the tricyclo[126.96.36.199(2,5)]dodeca-1,5-diene (TCDD)-Cl-2 system was studied by B3LYP/6-311+G(d,p) method and the configurations [reactants, 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 Cl-2 and TCDD. With a barrier of 22.362 kcal mol(-1) the CT-complex can be activated to an intermediate (INT) with energy 14.682 kcal mol(-1) higher than that of the CT-complex. The intermediate (INT) then transforms easily (barrier 5.102 kcal mol(-1)) into the final, N-type product. Accompanying the breaking of the Cl-Cl bond, C1-Cl, C5-Cl 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 the intramolecular skeletal rearrangement that is realized by the formation of the C2-C6 bond.