Many studies undertaken on urban catchments show, thanks to indirect approaches, that the contribution of eroded sewer sediments to pollution of combined sewer overflows is significant and highly organic. An in situ study of the erosion of sewer sediments has been implemented to validate those results with a direct approach and to observe the processes of erosion. Two experiments have been carried out on a 150 m length of combined sewer in “Le Marais” catchment in Paris, in order to determine the rate of erosion and the nature of the particles eroded by an injection of drinking water in the sewer system. Hydraulic and quality parameters have been measured in situ. Those injections have shown that the rate of erosion is important (maximum rate of 146 g/s) at each stage of the injection, which has been conducted in three stages with a maximum flow of 370 m3/h. The erosion does not only occur locally but happens along the entire length of the section even at low shear stresses (0.5 N/m2). The eroded particles are highly organic (VS=54–86%) and their loads in volatile solids, COD, BOD5 decrease as the flow increases. So, this work confirms, by direct measurements, that eroded sewer sediments are a significant source of organic matter that contribute to combined sewer overflow.