The OH addition to ethene has been modeled using ab initio quantum chemical calculations and classical transition state theory (CTST). The results agree with the hypothesis of Singleton and Cvetanovic (Singleton, D. L.; Cvetanovic, R. J. J. Am. Chem; Sec. 1976, 98, 6812) that the reaction is not elemental, and that it consists of a reversible first step involving the formation of a prereactive complex, followed by the irreversible formation of an addition adduct. The overall rate depends on the rates of two competitive reactions, i.e., the reverse of the first step and the second step, the former being more favored by an increase in temperature than the latter. Applying CTST to the proposed mechanism, we obtain an overall rate constant of 11.7 x 10(-12) cm(3) molecule(-1) s(-1), which agrees very well with the experimental results. New results for the activation energies of the OH addition to a series of substituted ethenes have also been obtained assuming that the above mechanism holds. The activation energies were calculated from projected second-order Moller-Plesset total energies obtained with the 6-311G** basis set. We find that a plot of these data vs the logarithm of the experimental rate constants has a correlation coefficient of 0.996. This seems to imply that the preexponential factor for the series should be approximately constant, in contradiction with the reported values for A. Moreover, a plot of our effective activation energies for OH addition vs the activation energies of a similar reaction, the addition of atomic oxygen to the same series of alkenes, also yields a good correlation. Indeed, for this reaction, the reported A factors are similar to each other. We suggest that the experimental Arrhenius parameters of the OH addition reaction need to be revised.