With the use of computer simulation (AQUASIM), the dynamic behavior of the two fluorescent whitening agents (FWAs) that are currently used in laundry detergents in Switzerland (DSBP, a distyrylbiphenyl, and DAS 1, a diaminostilbene) has been evaluated quantitatively for the River Glatt, a small highly polluted stream in Switzerland. The one-dimensional simulation was based on independently determined photolysis and sorption rate constants, and measured FWA concentrations at the beginning of the studied river section and in two major tributaries (effluents of sewage treatment plants). Concentrations of FWAs were 0.4-1.2 μg/L in river water and 2.9-5.7 μg/L in sewage, respectively, with very little diurnal variation. Direct photolysis and partitioning between aqueous phase and sediment and macrophytes were identified as the most important processes affecting the fate of DSBP and DAS 1 in the river. Simulation of FWA concentrations with photolysis as the only process yielded concentration traces with minima during daytime and maxima at night, which contradicted the observed uniform concentrations. By taking sediment/water exchange into account, good agreement between measured and modeled FWA concentrations was achieved. The observed removal of FWAs during transport (12 h residence time) was 31% and 12% for DSBP and DAS 1,respectively, corresponding to half-lives of 0.9 and 2.7 d, respectively, under cloudless summer skies.