Sediment cores from Lake Ketelmeer, a sedimentation area of the River Rhine were analyzed for priority pollutants (8 metals, 8 polycyclic aromatic hydrocarbons (PAHs), 7 polychlorinated dibenzo-p-dioxins, 10 polychlorinated dibenzofurans, 6 planar and mono-ortho polychlorinated biphenyls and 11 polychlorinated benzenes) in order to determine trends in the last 50 years. Post-depositional changes in pollutant levels were verified. Metal and PAH levels reflected the unchanged historical inputs. Concentrations were high in the late 1930s, low during the second world war and reached a maximum between 1955 and 1975. As an example, mercury levels changed from 3 to 2 mg/kg and reached 13 mg/kg in these successive periods. Levels in recently deposited sediment are ca. 2 mg/kg again. Some chlorinated aromatics proved to be persistent as well. For example, the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin had concentrations ≤10 ng/kg in the early 1940s, a maximum of ca. 400 ng/kg in the 1960s and again very low levels (≤10) in recently deposited sediment. On the other hand, post-depositional changes were observed for a few higher chlorinated dioxins and furans and several biphenyls and benzenes. This appeared upon comparison of concentrations in sediment layers collected recently and 20 years ago respectively and was possibly caused by microbial dechlorination. The historical inputs of these pollutants will be underestimated by the sediment core data.