The deuterium kinetic isotope effect (dkie) for the electrophilic bromination of ethylene-h(4) and ethylene-d(4) in methanol and dichloroethane (DCE) at 25 degrees C has been determined using mass spectrometry. The dkie's are inverse, that in methanol being k(H)/k(D) = 0.664 +/- 0.050 and that in DCE being k(H)/k(D) = 0.572 +/- 0.048. A product study of the bromination of trans-ethylene-d(2) in dichloroethane indicates that the addition is trans. Computations of the expected equilibrium deuterium isotope effect (EIE) for the process C2H4 + Br+ reversible arrow C2H4-Br+ using density functional theory indicate that the EIE is also inverse at K-H/K-D = 0.63. Detailed analyses of the molar partition functions and the zero-point energies for the various vibrational modes in the ground and ion states indicate that the major contributor to the EIE is the creation of a new mode in the ion, termed the CH2-symmetric twist, that arises from the loss of the rotational freedom about the C-C axis in ethylene. In the absence of this new mode, the computed EIE is normal, K-H./K-D = 1.12. The computations also indicate that the ion state undergoes very little rehybridization of the carbons, the sum of the H-C-H and H-C-C angles at each carbon being 357.3 degrees. A discussion is presented concerning the detailed sequence of events contributing to the reaction mechanism in both solvents and how each of these might contribute to the dkie.