Elevated levels of polychlorinated biphenyls (PCBs) have been detected in some Swiss watercourses in the past. The emission sources responsible for the contamination were initially unknown. There was thus a need to evaluate an appropriate method to measure the concentrations of PCBs and dioxins in long flowing sections of watercourses. This report evaluates the practicality of using sediment analyses and passive samplers in the aqueous phase to localise sources of PCBs and dioxins. It concludes that PDMS passive samplers are well-suited to this purpose because they enable reproducible determination of mean concentrations in the water phase over a long period for a particular location of a watercourse and are simple to use. Sediment analysis, however, is less suitable because sediments are mobile and their composition can vary with time and space. The two methods are described in detail in the report.