The objective of the study was to evaluate the Perfluorooctane Sulfonic Acid (PFOS) distribution pattern in the Western Scheldt estuary and the Belgian North Sea by measuring PFOS in organisms of different trophic levels, originating from different locations and in different fish (pelagic, benthopelagic). Furthermore, the investigation of time trends of the compound would enable us to have indications on how this compound behaves in the environment.The measurement of PFOS would be based in two different extraction protocols. However due too unforeseen reasons the acceptable results were much less than initially planted. That is why the present study deals mostly with the findings from the invertebrates. Based on those results there is PFOS contamination in the Western Scheldt, years after the production was phased out. Evidence is provided that there could be a PFOS pollution gradient in the Western Scheldt, towards the sea. So domestic and industrial sewage can be potential sources of PFOS. However further research is needed, since the pathways of PFOS exposure are unknown. We also confirmed the bioaccumulation potential of PFOS. This is a preliminary study that can be used for further research, in order to make more general conclusions about PFOS.