This paper, based on laboratory experiments, deals with the behaviour of metallic species Cu, Cd, Fe, Zn and Mn during the mixing of river and seawaters. The present study focused on the variations of metals speciation in both the dissolved phase and the suspended particulate matter. For the dissolved metals, a protocol using chelating resins permitted to perform trace metals speciation, i.e. fractionation into 'organic-metal' and 'inorganic-metal', and to preconcentrate them. The speciation of suspended particulate matter (SPM) was performed according to the sequential extraction procedure of Tessier et al. to partition particulate metals into five different fractions. All total metals had a conservative behaviour. However, there were great differences from one metal to another in the partitioning into dissolved and particulate phase. For iron, the metal was partly removed from the dissolved phase during the mixing, probably because of organic matter flocculation. Particulate iron was conservative because 90% of it remained in the 'residual' fraction of SPM. Copper had a conservative behaviour in both the dissolved and the particulate phases. The affinity for organic matter was well established for dissolved copper as well as for particulate copper, i.e. respectively 40% in 'organic' form and more than 60% in the 'organic' fraction. Manganese and cadmium were non-conservative: a clear enrichment of the dissolved phase was observed at low salinities (< 7.5). These metals were desorbed in inorganic form from the particulate phase and for a large part from the 'carbonates' fraction of the SPM.