Alkene halogenation belongs to a class of reactions known to be extremely sensitive to the environmental effect. In this article we discuss different aspects of the problem through ab initio calculations. Discrete and continuum models for the solvent are employed to analyse equilibrium and non-equilibrium solvation effects on the reaction of ethylene with molecular bromine. It is shown that there are two mechanisms that lead to transition states of different symmetry. One mechanism is found in gas phase and non-polar solvents. The second one, that leads to the typical C2v transition state, holds in medium polar to very polar media. In water, the solvent molecules participate actively to the reaction coordinate. In this solvent, non-equilibrium solvation effects are shown to be substantial and larger that those previously reported for the SN2 reaction.