Ab initio calculations were carried out for the reaction of Br-2 with ethene, propene, isobutene, fluoroethene, chloroethene, (E)-1,2-difluoroethene, and (E)-1,2-dichloroethene. For ethene the calculations were also carried out for the reaction with 2Br(2). Geometries were optimized at the HF, MP2, and B3LYP levels using the 6-31G(d) and 6-31+G(d) basis sets where for Br both the standard 6-31G and the Binning-Curtiss bromine basis sets were used. Energies were also calculated at the G3MP2 and G3MP2B3 levels. For a single Br-2 one mechanism involves a perpendicular attack by Br-2 to the C=C bond, and a second mechanism consists of sidewise attack by Br-2. Alkenes can react with 2Br(2) via several mechanisms, all leading to the dibromo product. The most likely pathway for the reaction of ethene and 2Br(2) involves a trans addition of a Br atom from Br-3(-) to one of the bromonium ion carbons. Activation energies, free energies, and enthalpies of activation along with thermodynamic properties (Delta E, Delta H, and Delta G) for each. reaction were calculated. We have found that the reaction of ethene with 2Br(2) is favored over reaction with only Br-2. There is excellent agreement between the calculated free energies of activation for the reaction of ethene and 2Br(2) and experimental values in nonpolar aprotic solvents. However, the free energies of activation for the reaction with a single Br-2 agrees well with the experimental results for polar protic solvents only when the reaction is mediated by a solvent molecule. A kinetic expression is proposed that accounts for the difference between bromination of alkenes in protic and nonprotic solvents. Some previously unknown heats of formation are reported.