Ozone reacts with various halogen compounds (chlorine, chloramines , bromine , and bromide) which may be present in water. Chlorine produces Cl−and ClO−3; chloramines produce NO−3and Cl−. Bromide produces bromine , which acts as a secondary disinfectant, but can generate haloforms and interfere in O3determinations. Brominepartly regenerates Br causing a catalytic destruction of O3. The kinetics of these reactions are described and interpreted with respect to their importance in water treatment.