1-Chlorobenzotriazole adds rapidly to olefins to give both 1- and 2-(2-chloroethyl)benzotriazoles in good yield, with the latter predominating. N.m.r. is particularly useful for diagnosis of the product structures. The relative reactivities of olefins, the Markovnikov nature of the orientation, the stereospecific trans-addition (to cis-and trans-but-2-ene), and the absence of an induction period are all as expected for electrophilic addition of Cl⁺ followed by nucleophilic attack by the ambident benzotriazole anion. In further agreement with this, reaction with cyclohexene is faster in acetic acid than in less polar solvents and gives predominantly 2-chlorocyclohexyl acetate. Under u.v. irradiation a competing radical process intervenes to increase the proportion of 1-substituted benzotriazole.