The thermochemistry and kinetics of the H3Si.+ C2H4 -> H3Si-(CH2CH2)-H-. reaction were studied using UQCISD(T)/aug-cc-pVTZ// UQCISD/aug-cc-pVDZ. Our results show that the Arrhenius activation energy, E-a, for the reaction is 3.3 kcal/mol, substantially lower than the 6 kcal/mol value determined experimentally by Loh et al. [S.K. Loh, D.B. Beach, J.M. Jasinski, Chem. Phys. Lett. 169 (1990) 55]. Loh et al.'s high E,, was the result of an estimate of the experimental A-factor that was two orders of magnitude too high. Our calculated rate constant of 1.41 x 10(6) M-1 s(-1) is in very good agreement with the upper bound of 1.81 x 106 M-1 s(-1) determined by Loh et al. A study of the thermochemistry and kinetics of the addition reaction using 15 density functional theory methods with 6-31G(d) basis sets indicates that BHandHLYP predicts results in best agreement with high-level theory. Published by Elsevier B.V.