River waters and wastewater treatment plant effluents contain hydrophobic organic compounds (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorobiphenyls (PCBs) which are persistent, bioaccumulative and dangerous for the environment. The biological risk assessment of HOCs requires the estimation of their bioavailable fraction in addition to the total contamination of the media. The bioavailability of HOCs is dependent on the aquatic environment characteristics. In particular, HOCs may be bounded by dissolved organic matter (DOM) making them more mobile and less biodegradable by biological/chemical processes. The presence of DOM also usually reduces the bioavailability of HOCs. The generally accepted assumption is that pollutants that are bound to DOM are too large to cross biological membranes. An artificial device (Semi-Permeable Membrane Device (SPMD)) has been developed to evaluate bioavailable fractions of HOCs [1,2]. It is a long, flat, polyethylene (PET) tube containing lipid (triolein). The 10 Å pore diameter of PET allows contaminants to pass through, in the same way as the membranes of animal cells and the triolein was chosen to be like lipids in biological tissues. We tested the influence of DOM characteristics upon the HOCs SPMD-availability. SPMD were deployed in several river waters (up and downstream a big town) and in wastewater treatment plant effluents. The priority 16 PAHs and 7 PCBs were analyzed in suspended particles, filtered waters and in SPMD. DOM was characterised for its hydrophobicity (by organic carbon analysis of XAD8 and XAD4 isolates), aromaticity (by SUVA254, ie UV254/ DOC), biodegradability and molecular weight. The total HOCs contamination increased downstream the river ; wastewater effluents were proved to be a source of SPMD-available HOCs. Relationships between the characteristics of organic matter and the SPMD-available fraction have been established. The SPMD-available fraction is negatively related to DOM hydrophobicity and molecular weight.