New methods for the determination of reactive bromine and chlorine species are presented. Hypobromous acid (HOBr) and all three bromamines species (NH2Br, NHBr2, NBr3) are analyzed as a sum parameter and hypochlorous acid (HOCl), monochloramine (NH2Cl) and chlorine dioxide (ClO2) can be determined selectively. However, no distinction is possible between HOCl and the active bromine species. The bromine and chlorine species react with ABTS (2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid-diammonium salt) to a green colored product that is measured at 405 or 728 nm. Free chlorine and NH2Cl can be measured in the presence of ozone. The method is therefore suitable if combinations of disinfectants are used, such as chlorine/chlorine dioxide or chlorine/ozone. In natural waters, the method provides a detection limit for all chlorine/bromine species of less than 0.1 muM. The colored reaction product is very stable and allows a fixation of the chlorine/bromine species in the field and subsequent determination of the absorption in the laboratory. (C) 2000 Elsevier Science Ltd. All rights reserved.