Determining the bioavailability of sediment-associated compounds is an essential but difficult part of the modern environmental risk assessment process. Numerous environmental factors, e.g. temperature, chemical contact time, chemicals present in the sediment, chemical and sediment properties modify bioavailability of model compounds, but this change in bioavailability is partially based on the changes in chemical's desorbing fractions. However, bioavailable fraction can not be solely interpreted by reference to the size of the desorbing fraction. For example the rapidly desorbing fraction did not totally describe the bioavailable fraction of "superlipophilic" polychlorinated dibenzo(p)dioxins and -furans, polychlorinated diphenyl ethers in field-contaminated sediments. In addition, the organisms' sediment ingestion may modify the bioavailable fraction in a way that is not taken into account in the bioaccumulation models. Regardless of this, results indicate that the freely dissolved pore water concentration can give a better estimation for the bioavailable fraction than do total concentration and thus could improve the realism of site-specific ecological risk assessment.