HOBr can be formed in various oxidation processes in engineered and natural systems. The rate of HOBr reduction by H2O2 is decisive to avoid formation of brominated organic compounds and bromate during ozone or hydrogen peroxide-based advanced or naturally occurring oxidation processes. From the pH dependence of this rate that was determined by stopped-flow measurements we conclude that either OBr- and H2O2 or HOBr and HO2- react with each other. Assuming that either one of the reactions takes place the corresponding second-order rate constants were determined to be k(OBr-,H2O2) = (1.2 +/- 0.2). 10(6) M(-1) s(-1) and k(HOBr,HO2-) = (7.6 +/- 1.3). 10(8) M(-1) s(-1). Mechanistic considerations lead to the conclusion that a nucleophilic attack of HO2- on HOBr must be the dominant reaction in the system. From the determined rate constants it can be estimated that the half-life for HOBr is less than a few seconds for a H2O2 concentration of 0.1 mg L(-1) (3 mu M) at pH 8. However, at a lower pH of 5, as encountered in cloud waters. the half-life of HOBr is several hours for the same H2O2 concentration. (C) 1997 Elsevier Science Ltd.