Kinetics, products, and pathways of 17 alpha-ethinylestradiol (EE2) transformation during chlorination of water with and without bromide (Br-) were studied to evaluate the effect of chlorination on the fate of steroid estrogens. Br- can be important for the fate of EE2 during chlorination because of a rapid conversion of Br- to bromine by chlorine and a similar to 1000 times higher reactivity of bromine than chlorine toward EE2. Experimental data and model calculations for EE2 chlorination show that 4-chloro EE2 and 2,4-dichloro EE2 are the major initial transformation products in absence of Br-. With increasing concentration of Br-, the bromo-analogous of the chloro EE2s becomes significant as additional initial transformation products. Those initial transformation products of EE2 are generally quickly further transformed to products with a destroyed phenolic moiety. Transformation of EE2 during chlorination becomes considerably faster at Br- concentrations higher than 0.25 mu M (20 mu g L-1). However, in presence of model dissolved organic matter such as glycine or phenol, the accelerating effect of Br diminishes due to a more rapid consumption of bromine than chlorine by these compounds. A kinetic model was developed which allows to predict EE2 transformation during chlorination of natural waters and wastewaters in presence of Br- and low concentrations of ammonia (<10 mu M). It could be demonstrated in these systems that bromine produced from Br- is mainly responsible for EE2 transformation.