Ecotoxicological tests with benthic organisms are recommended to assess the toxicity of hydrophobic organic contaminants such as polycyclic aromatic hydrocarbons that strongly adsorb on sediments. As a matter of fact, many routes of exposure including ingestion from sediments are taken into account with bioassays. However, chemical partitioning and toxicity of these nonpolar organic contaminants could be affected by the organic matter of the sediments. The purpose of this study was to assess the influence of both quality and quantity of the organic matter on the bioavailability of phenanthrene for benthic organisms, Chironomus riparius and Hyalella azteca. Formulated sediments were useful models to determine the influence of the organic matter excluding any other change: 2 to 8% TOC formulated sediments were used with 3 different organic matters. Acute (survival) and chronic (growth, size) endpoints were investigated. Bioassays were carried out until adult stage for both organisms. Toxicity parameters (NOECs, LC50) were derived for survival of the organisms. After 10 days (chironomids) or 14 days (Hyalella) exposure, survival of the organisms were below 44% at 300 mg/kg phenanthrene (dry weight). However, there was no significant differences for the growth of the survival midges at any concentration. With hardly any survivals of chironomids after 28 days exposure, toxicity on adults was higher than previously observed on 10 days larvae. Both the quality and the quantity of the organic matter influenced acute toxicity results whereas partitioning of phenanthrene was not affected by the organic matter. The toxicity results showed that equilibrium partitioning theory that assumes that benthic organisms are primarily exposed to contaminants via the water phase could not be applied in that case to assess the risk that phenanthrene pose to sediments in the environment.