It is now well established that hydrocarbons are recognised like hazardous compounds, especially polynuclear aromatic hydrocarbon (PAH) [Bomboi and Hernandez, 1991; Hermann R, 1981], Urban runoff is a non negligible source of hydrocarbon that may damage the quality of receiving river. In this report, we present the first results of a study of the impact of combined sewer overflows (CSOs), during summer 1995, on the distribution of n-alkanes (C11-C37 ) in the Seine river. Figure 1 shows the site of Chatou, where suspended solids (SS) were sampled just before and during a week period following the moderate CSO event of September 7, 1995.