The understanding of transport and dispersion of combined sewer overflows (CSOs), containing both dissolved and paniculate matters, is a major component of a research program dealing with the assessment of impacts of CSOs on the Seine River. This study has combined field measurements and the development of modelling tools. The first experimental results stress the fact that predictions of the behaviour of dissolved matter require the use of a fully two-dimensional hydraulic model, because the dispersion process seems to strongly depend on geometrical features of the river - meanders and the differential advection they induce. Concerning the paniculate matter, it appears that different factors govern its downstream transport, namely, settling properties, critical shear stress, and vertical diffusivity. These factors may produce forces on particles of the same order of magnitude, but in opposite directions. Special experiments are needed to determine precisely how these forces combine.