This paper presents a theoretical study of the addition of (HF)2 to ethylene using ab initio methods with the 3-21G basis set. Thermodynamical calculations to obtain ΔG°298 values were also made. Comparison with the bimolecular addition of HF to ethylene shows a larger stabilization of the transition state obtained, reflected in a decrease of the potential barrier. This fact, along with an analysis of the energy components and the mechanism, allows us to assert the catalytic activity of the second HF molecule. From the thermodynamics of the reaction, the need for a “bimolecular” collision between (HF)2 and ethylene and not a termolecular one is deduced, and the ability of this mechanism to explain data in the gas phase and in nonpolar solvents is sustained. © 1986, American Chemical Society. All rights reserved.