The long disputed mechanism for the F-2 addition to ethene is elucidated by density functional theory calculations. With hybrid functionals and a large basis set, DFT provides an excellent description for the dissociation of ground state F-2, and a reasonable account for the F-2.C2H4 van der Waals complex, which makes it possible for the modeling of reactions between elemental fluorine and organic molecules. The attack of F-2 on ethene first produces a diradical intermediate, which then dissociates into two radicals CH2F-CH2. and F.. The first step is exothermic with a low barrier around 1.8 kcal/mol, and the exothermic energy is more than enough to overcome the barrier in the second step for the homolysis of the dangling F-F bond in the diradical, although the presence of solvents and matrices environment could stabilize the diradical. Our calculations provide a coherent framework to understand this reaction not only in the gas and solution phases, but also in the matrices environment where mode-specific enhancement is observed for the addition process induced by infrared radiation. (C) 2004 American Institute of Physics.