Abstract : The discharge of combined sewers to the river Seine during summer storms is one of the major problems the Water Authorities of the Parisian urban area have to deal with. A better characterization of the discharged pollution will allow a more accurate evaluation of the oxygen depletion in situ. The composition of organic matter transported by combined sewers during dry and wet weather was studied in an experimental catchment of city Boulogne, near Paris. The concentrations of organic carbon were higher during dry weather than during wet weather, due to dilution by run off. The dry weather organic matter was thereby significantly more degradable than during wet weather. The degradability decreased with rainfall intensity. During the day, the dry weather flows and the concentration of pollutants were higher than during the night. The flux of pollutants discharged during the day was therefore higher than for an equivalent rainfall during the night. The discharged waste water has an effect not only on oxygen depletion but also on phytoplankton. During laboratory essays with natural phytoplankton, we have observed severe effects of filtered effluent on the primary production. Depending on CSO, an in situ dilution of 20% of waste water may decrease the net production by 30%. By this way, the inhibition may lower the oxygen deficit by a few percent. An important part of this effect is due to heavy metals like zinc. Their toxicity will depend of the way they are complexed. The degradation of organic matter in situ was characterized by means of study of waste water masses issued from the major overflow of Parisian area. The pollution plug was followed for more than 20 km using conductivity and oxygen measurements. Conductivity and ammonium were confirmed to be conservative and therefore good tracers of waste waters. We have observed that the oxygen deficit was related to the mass of discharged BDOC, rather than to the suspended solids or POC, which was especially due to high settling velocity of the discharged solids. However an excess of DOC of unknown origin, was observed in situ after the overflows. The oxygen deficit was mainly caused by allochtonous bacteria larger than 1 µm. Their specific activity was 3 to 4 times higher, compared to the small autochtonous bacterial population. Most of the microbial processes evaluated had higher rates than suggested by previous works. The results obtained allowed a better modeling of the CSO's impacts in the river Seine.