Hypochlorous acid (HOCl) is typically assumed to be the primary reactive species in free available chlorine (FAC) solutions. Lately, it has been shown that less abundant chlorine species such as chlorine monoxide (Cl2O) and chlorine (Cl-2) can also influence the kinetics of the abatement of certain organic compounds during chlorination. In this study, the chlorination as well as bromination kinetics and mechanisms of 12 olefins (including 3 aliphatic and 9 aromatic olefins) with different structures were explored. HOCl shows a low reactivity towards the selected olefins with species-specific second-order rate constants < 1.0 M(-1)s(-1), about 4-6 orders of magnitude lower than those of Cl2O and Cl-2. HOCl is the dominant chlorine species during chlorination of olefins under typical drinking water conditions, while Cl2O and Cl-2 are likely to play important roles at high FAC concentration near circum-neutral pH (for Cl2O) or at high Cl- concentration under acidic conditions (for Cl-2). Bromination of the 12 olefins suggests that HOBr and Br2O are the major reactive species at pH 7.5 with species-specific second-order rate constants of Br2O nearly 3-4 orders of magnitude higher than of HOBr (ranging from 0.01 to 10 3 M(-1)s(-1)). The reactivities of chlorine and bromine species towards olefins follow the order of HOCl < HOBr < Br2O < Cl2O approximate to Cl-2. Generally, electron-donating groups (e.g., CH2OH and CH3-) enhances the reactivities of olefins towards chlorine and bromine species by a factor of 3-10(2) , while electron- withdrawing groups (e.g., Cl-, Br-, NO2-, COOH-, CHO-,-COOR, and CN-) reduce the reactivities by a factor of 3-10(4). A reasonable linear free energy relationship (LFER) between the species-specific second-order rate constants of Br2O or Cl2O reactions with aromatic olefins and their Hammett sigma(+) was established with a more negative rho value for Br2O than for Cl2O, indicating that Br2O is more sensitive to substitution effects. Chlorinated products including HOCl-adducts and decarboxylated Cl-adduct were identified during chlorination of cinnamic acid by high-performance liquid chromatography/high resolution mass spectrometry (HPLC/HRMS). (c) 2020 The Author(s). Published by Elsevier Ltd.