The polycyclic musk fragrance compounds HHCB (1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta-(g)-2-benzopyran; trade name, e.g. galaxolide®) and AHTN (7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene, trade name, e.g. tonalide®) and the transformation product of HHCB (HHCB-lactone) were analysed in surface water samples and sewage treatment plants (STP) effluents in the Ruhr megalopolis. The STPs were the dominant source for these pollutants. In the part of the river where the drinking water is extracted from the river, about 60 ng L⁻¹ HHCB, 10 ng L⁻¹ AHTN and 20–30 ng L⁻¹ HHCB-lactone were found as typical riverine concentrations, while none of the compounds were detected near the spring of the river. On the other hand sewage treatment plant effluents exhibited concentrations up to 600 ng L⁻¹. The STP’s effluent resulted in elevated concentrations in some parts of the river and in the lakes into which they discharge. As some of the plants emit HHCB-lactone with a significantly changed enantiomeric pattern, biotransformation of HHCB to HHCB-lactone occurs in some waste water treatment plants operating with activated sludge. In those parts of the river where no relevant discharges of waste water or fresh water takes place neither the concentration nor the pattern changes significantly. This holds true especially for the HHCB versus HHCB-lactone ratios which indicates degradation less than 15% of the HHCB inventory in the river Ruhr itself. In other rivers, such as the Rhine, higher levels of HHCB-lactone in comparison to HHCB were detected (ratio 1 ∶ 1).