Sediment deposited in the Tilbury Basin exhibited two distinct zones where contamination with organochlorine insecticides (OCL) and polychlorinated biphenyls (PCB) differed by up to an order of magnitude. The upper section of the core had an average concentration of 39 ng g-1 PCB and ΣDDT (sum of concentrations of DDT, DDD and DDE) 47 ng g-1 whereas in the lower section these were 309 and 87 ng g-1 respectively. This difference was attributed to improvements in waste water treatment technology, specifically the extension of activated sludge treatment at Beckton and its introduction at Crossness sewage treatment works (STW) between 1959 and 1964. At the time Beckton alone was the largest STW in Europe, treating waste from a population of 2.82 million in 1961, with a flow of 1.14 × 106 m3 d-1 through the works making it the largest tributary of the Thames. The abruptness of change in pollutant concentrations observed in cored sediments at -5.82 m below Ordnance Datum Newlyn (OD) was linked to dredging activities within the basin and did not reflect rates of temporal changes in inputs. There was evidence to suggest that dechlorination of PCB had occurred within the deposited sediments. This was expressed as a change in the ratios of lower chlorinated (tri– and tetra–) congeners relative to those with 5 or more substituted chlorine atoms. An average ratio of 0.23 from the sediment surface to -2.29 m OD changed to 0.32 and subsequently increased to 0.62 between -5.82 m to -9.55 m OD. A number of factors may account for changes in microbial dechlorination activity. However, the possibility that changes in input sources were responsible for such effects cannot be discounted.