The mechanism of the reaction F~2+2HCl→2HF+Cl~2 has been investigated using density functional theory (DFT) at the B3LYP/6- 311G^*^* level. The transition states for all the possible reaction paths are obtained and verified by vibrational analyses and IRC calculations. By comparing the calculated barriers, it is found that the largest barrier for the mechanism of molecule-molecule interaction is 150.63kJ.mol^-^1. The corresponding barrier for the mechanism in which F~2 is cleavaged first before the resulting F atoms react with the other reactants is 154.82kJ. For the reaction F+HCl→HF+Cl, two types of transition states are located. One is linear, the another is triangle and it is more stable. Between two transition states for the reaction HCl+Cl→H+Cl~2, the linear one is more stable.

Key words: HYDROGEN CHLORIDE, FLUORINE, REACTION MECHANISM, TRANSITION STATE THEORY