An experimental catchment area was set up by CEREVE in the centre of Paris, in order to obtain an accurate description of the pollution caused by urban wet weather flows at different locations within the combined sewer system, and hence to estimate the contribution of runoff, waste water and sewer sediments to the total pollution. The results obtained show a change in quality between the runoff entering the sewer network and the combined storm water flow at the sewer's outlet. In particular, an increase was observed in the concentrations of SS, VSS, COD, BOD5, Cu and in the settling velocities of particles. The assessment of the contribution of different sources to the pollution showed that the organic layer at water-sediment interface in the sewer is the main source of wet weather flow pollution for suspended solids, volatile solids, particle-bound COD and BOD5, and copper. For cadmium, lead and zinc, it was shown that roof runoff constitutes the main source of wet weather flow pollution in 'Le Marais' catchment, due to the corrosion of the roofing materials. (C) 2004 Elsevier B.V. All rights reserved.