Eleven wastewater treatment plants (WWTPs) collecting various types of wastewaters (combined and separate sewers, household and industrial wastewaters) have been studied from 2005 to 2007. At each plant, labile, dissolved and total trace metals were sampled (24 hour composite samples) and analyzed (ICP-AES), at the inlet and at the outlet. Special emphasis was given to three plants, on which exhaustive sampling campaigns were conducted: labile, dissolved and total Polycyclic Aromatic Hydrocarbons (PAHs) and trace metals were sampled and analyzed at every stage of the treatment. Labile fractions were obtained by deploying respectively Semi-Permeable Membrane Devices and Diffusion Gradient in Thin films for PAHs and metals. The data allow to quantify PAHs and metals fluxes at the inlet and outlet of the WWTP and to evaluate their significance regarding environmental fluxes. Moreover, the efficiency of each treatment stage will be discussed, depending on the various species. Special emphasis will be given on the fact that labile and dissolved forms of contaminants are much less retained in the plants by conventional treatments than particulate ones. Moreover, as far as metals are concerned, some additional tertiary treatments (used for flocculating suspended matter) constitute a significant source of dissolved metals. These results are likely to stimulate the discussion on the ecotoxicological impact of treated wastewaters on aquatic ecosystems, in so far as labile contaminants are thought to be the most bioavailable and possibly toxic- ones.