The reactions of hydroxyl radical with ethene, fluoroethene, and chloroethene have been studied by quantum chemical methods. Reactants, prereaction complexes, transition-state structures, and products were optimized and vibrational frequencies were calculated at the UMP2/6-311+G(2d,p) level. Transition-state structures are significantly different from the prereaction complexes formed on the reactant side of the MEP. The convergence of barrier heights and reaction enthalpies has been systematically investigated with respect to the size and quality of basis set and the treatment-of correlation energy. The best agreement with experimental results is found at the MP2/aug-cc-pVTZ level of theory. Regioselectivity is discussed in terms of two properties of the radical and the investigated alkenes. The first factor is the relative spin density in the (3) pi pi* state of the alkene. The second factor is the relative strengths of the product C-O bond, i.e., relative stability of the corresponding radical product. In the case of fluoroethene these two effects oppose each other and regioselectivity is negligible. In the case of chloroethene spin density is the dominant factor and the addition of OH radicals to the unsubstituted carbon atom is preferred.