**Abstract** : The dimensionless M(V) curves indicating the distribution of pollutant mass vs volume in stormwater discharges are used to compare pollutant discharges from different rainfall events and catchments. The M(V) curves are very variable and dispersed for each pollutant and for a series of rainfall events in a given catchment. From the analysis of 197 rainfall events in 12 separate and combined sewer systems, the following results can be extracted: - in separate sewer systems, 80% of the total pollutant mass is transported in the first 74% of the total volume for 50% of the rainfall events; - in combined sewer systems, 80% of the total pollutant mass is transported in the first 79% of the total volume for 50% of the rainfall events. The characteristics of the M(V) curves depend on the pollutant, the site, the rainfall event and the functioning of the sewer system. No general multi-regression relationships can be established to explain their shape and their variability because of the complexity of the phenomena involved and the multiplicity of influencing factors. A new definition of the first flush is derived from the analysis of the M(V) curves, allowing a non-ambiguous quantification of a phenomenon which, up to now, was presented in a principally descriptive or qualitative manner. It is assumed that there is a significant first flush if at least 80% of the total pollutant mass is transported in the first 30% of the volume discharged during rainfall events. Previous definitions proposed by other authors are discussed with regard to the treatment of stormwater discharges. However, the concept of the first flush can not be used alone to establish a reliable methodology to design treatment facilities. Additional knowledge and information are necessary. From this point of view, the analysis of experimental M(V) curves is appropriate to base a design methodology which takes into account the intrinsic variability of the phenomena.The dimensionless M(V) curves indicating the distribution of pollutant mass vs volume in stormwater discharges are used to compare pollutant discharges from different rainfall events and catchments. The M(V) curves are very variable and dispersed for each pollutant and for a series of rainfall events in a given catchment. From the analysis of 197 rainfall events in 12 separate and combined sewer systems, the following results can be extracted: - in separate sewer systems, 80% of the total pollutant mass is transported in the first 74% of the total volume for 50% of the rainfall events; - in combined sewer systems, 80% of the total pollutant mass is transported in the first 79% of the total volume for 50% of the rainfall events. The characteristics of the M(V) curves depend on the pollutant, the site, the rainfall event and the functioning of the sewer system. No general multi-regression relationships can be established to explain their shape and their variability because of the complexity of the phenomena involved and the multiplicity of influencing factors.