A series of 5-bromomethyl-3-substituted Δ²-isoxazolines were prepared from the corresponding β,γ-unsaturated oxime by the addition of bromine in dichloromethane. The mechanism of this reaction involves addition of the bromine to the alkene and formation of the bromonium ion, followed by intramolecular addition of the hydroxyl group to the bromonium ion. The synthesis of the β,γ-unsaturated oxime and the mechanism of the title transformation will be discussed.