In bioavailability sudies, the biota sediment accumulation factor (BSAF) is invoked to describethe thermodynamic partitioning of a hydrophobic organic contaminant (HOC) between the organism lipid and the organic carbon fraction of the sedimentary matrix and accounts for differences in bioavailability among sediments. Bioaccumulation experiments were performed with Lumbriculus variegatus and Diporeia species exposed in seven sediments dosed with2,4,5,2′,4′,5′-hexachlorobiphenyl (HCBP) and benzo[a]pyrene (BaP) or pyrene (PY) and 3,4,3′,4′-tetrachlorobiphenyl (TCBP). The BSAF values for the nonplanar HCBP were consistent with equilibrium partitioning theory (EQP) and averaged 2.87 for L. variegatus and 1.45 for Diporeia, while the BSAF values for the planar compounds (BaP, PY, TCBP) were generally lower than estimated from EQP (<1). Correcting the BSAF values of the planar compounds for enhanced sorption due to black carbon improved the BSAF values for L. variegatus, generally resulting in values consistent with EQP, but substantial variation remained for Diporeia. The BSAF values for the planar compounds showed significant positive correlations with plant-derived carbon in sediments (lignin and pigments) but were more consistent for L. variegatus than for Diporeia. These correlations imply that compounds sorbed to plant-derived carbon are more bioavailable since this material is more likely ingested providing a second exposure route.