||Sediments are considered to be the sinks of trace metals including mercury, and acquiring trace metals from sediments is found to be the dominant route of trace metal uptake for many aquatic organisms. In this study, effects of geochemical and biological factors on the bioavailability of mercury (including inorganic mercury or Hg and methylated mercury or MeHg) in sediments were investigated. Contact time between mercury and sediments could decrease the mercury bioavailability through changing the mercury solid speciation. Most Hg and MeHg are associated with organic matters (OM) but not inorganic particles in oxic sediments, and the association could increase or decrease Hg bioavailability depending on the contact time. In anoxic sediments not only FeS but also FeS2 was important in controlling the partitioning and bioavailability of Hg in anoxic sediments. Hg in sediments can be partitioned into two pools of different mobility (operationally defined) depending on Hg concentrations in sediments. Hg solid speciation (or geochemical fractionation) has great effects on the bioavailability of Hg in sediments, and could be used as predictors of Hg bioavailability. Cysteine and other thiol groups are the key binding sites of Hg and MeHg in organic matters, in sediments and in digestive fluids of deposit-feeders, which greatly influence the sorption and bioavailability of Hg and MeHg in sediments. While both free amino acids and proteins contribute to the extraction of Hg by gut juices from sediments, proteins are the key extraction agents for MeHg. Bacteria play an important role in the partitioning and bioavailability of Hg and MeHg in sediments. We also found that digestive solubilization (using either original digestive fluids of organisms or mimic ones) is an easy while dependable method to quantify mercury bioavailability to animals.