Kinetic measurements on the rate of bromination of a series of arylidene arylhydrazines have been made in 70% aqueous acetic acid at 20°, by following the diffusion current of bromine (initially ca. 10–5 M) at a rotating platinum electrode. Substituents in the benzylidene ring have a smaller effect (ρ=–0·62) than those in the other ring (ρ=–2·2). This has been rationalised in terms of an intermediate stabilised by considerable charge delocalisation.