Epidemiological studies have shown an association between chlorination byproducts in drinking water and certain cancers in humans. However, the lack of consistency among studies prevents the establishment of a dose-response curve. This is partly due to uncertainties regarding exposure measurement, which is complicated by the number of exposure pathways involved and variations in concentrations within water systems. The study presented in this report describes the trends in three families of chlorination byproducts (trihalomethanes, haloacetic acids, and haloacetonitriles) observed in four French water systems in 2006 and 2007. Regarding trihalomethanes, whose concentrations double on average between the plant and the tap, a model predicting concentrations in the water system was developed. The five input variables are the concentration of trihalomethanes and residual free chlorine at the plant outlet; two variables indicating the reactivity of organic matter (specific UV absorbance or SUVA, and free chlorine consumption in the treatment plant prior to distribution); and the hydraulic retention time in the water distribution system. French regulations impose a limit value for trihalomethanes in drinking water, and most monitoring is conducted at the plant outlet. Applied to these measurements, the developed model would enable better monitoring of French citizens’ exposure. (R.A.)