The bimolecular reaction mechanism for adding hydrochloric acid to ethylene is studied within a number of contemporary quantum chemical models. The transition state structure and energy is examined in detail, and high-level calculations support a picture of an intimate association of a chloride anion and a bridged ethyl cation, with some covalent bonding retained between chlorine and hydrogen. A tunneling correction of 1 kcal/mol in the reaction barrier is obtained by the Bell equation. The methods employed include configuration interaction (CI) and gradient-corrected density functional theory (DFTG), with a range of basis sets. High-end CI and DFT methods perform equally well with respect to enthalpies of reaction and activation, when used in connection with large bases. However, when bases of double-zeta plus polarization quality are used, an unfortunate accumulation of errors makes DFTG inferior. This is in contrast to the excellent values for a number of ground state properties of relevance to the reaction barrier which are obtained at this level of theory. A recently proposed extrapolation procedure for CI energies (PCI-80) is shown to fail for the electron affinity of chlorine, leading to an exaggerated estimate of the barrier for the title reaction. The single-particle description of the rr bond in ethylene converges very slowly with the number of basis functions, and this affects the reaction enthalpy as computed by DFT and CI methods to a similar degree.