Step 1: Initiation

Once the pre-initiation step involving NBS produces small quantities of Br₂, the bromine molecules are homolytically cleaved by light to produce bromine radicals.

Step 2: Propagation

One bromine radical produced by homolytic cleavage in the initiation step removes an allylic hydrogen of the alkene molecule. A radical intermediate is generated, which is stabilized by resonance. The stability provided by delocalization of the radical in the alkene intermediate is the reason that substitution at the allylic position is favored over competing reactions such as addition at the double bond.

The intermediate radical then reacts with a Br₂ molecule to generate the allylic bromide product and regenerate the bromine radical, which continues the radical chain mechanism. If the alkene reactant is asymmetric, two distinct product isomers are formed.

Step 3: Termination

The radical chain mechanism of allylic bromination can be terminated by any of the possible steps shown below.