The Community strategy against pollution of surface water control policy was set out by the European Water Framework Directive 2000/60/EC which lays down new procedures for the identification of substances and development of control measures. After the decision n° 2455/2001/EC of the European Parliament and of the Council of 20 November 2001 setting out the first list of 33 substances or groups of substances that have been prioritized for action at community level, the Commission adopted on 17 July 2006 a Directive setting environmental quality standards for the priority substances which Member States must achieve by 2015, to ensure a "good chemical surface water status". Prior to achieve this objective by identifying possible reduction measures, an accurate knowledge on these substances at the different stages of the urban water cycle is required. Though some studies were launched to identify the occurrence and the significance of priority pollutants (PPs) in wastewaters (Rule et al., 2006ab; Palmquist and Hanaeus, 2005), no study focused on the quality of wet weather flows and combined sewer overflows (CSOs) within combined sewers, more especially for a heavily urbanized area such as Paris (France). Due to the lack of available information on the PP levels within combined sewers during wet periods, the OPUR research program (Observatory of Urban Pollutants in Paris, France) launched this study. Its objectives are i) to assess the priority pollutant concentrations, ii) to compare these concentrations to those observed in wastewater and runoff, and finally iii) to compare the measured concentrations to the Environmental Quality Standards (EQS). This survey constitutes one of the first studies in Europe to report data for a variety of priority substances in CSOs. A large range of PPs in CSO was observed, and most of them present concentrations in the μg.l-1 range, or even up to 10 μg.l-1. These results are of critical importance since the data generated for a predetermined number of hazardous substances may be used in the future to identify PPs of potential significance and to elaborate strategies for new monitoring programmes for both CSOs and receiving waters.