Bioavailability and uptake of trace metals by benthic animals are often assumed to be limited by authigenic sulfide minerals because of their low metal solubilities and reactivities under sedimentary conditions. However, digestive processes and gut conditions such as Eh, pH, and enzyme or surfactant activity, can affect the release of ingested metals in the gut and control uptake. In a series of laboratory experiments with the deposit-feeding clam, Macoma balthica and the suspension-feeding mussel, Mytilus edulis, we assessed assimilation efficiencies (AE) of radioisotopes of Ag, Cd and Co associated with acid-volatile sulfide (AVS), iron oxide (re-oxidized AVS), and reduced and oxidized natural sediment. To evaluate controls on AE, we measured the gut passage time (GPT) of ingested particles, gut Eh, pH, and extraction of Ag, Cd, and Co from particles into "gut juice." In general, the overall trends of AEs and metal extraction were Co > Cd ≥ Ag. AEs, metal extraction, and GPTs were higher in M. balthica than in M. edulis in most cases. M. balthica tended overall to take up metals more readily from oxidized than reduced natural sediment, whereas M. edulis did the opposite for Co and Cd. AEs of metals in reoxidized AVS (Fe-oxides) were generally similar to oxic sediment (Ag being the exception for M. edulis). In M. balthica, there was no significant difference in AEs from AVS and Fe-oxide particles for Cd (14 -20%) or Co (27-35%), but AEs for Ag from AVS particles were greater in large clams (28%) than small clams (15%). There were generally poor correlations between AEs of metals and metal release in gut juice. Low pH and moderate reducing conditions facilitated dissolution of AVS- and iron oxide-bound metal in the guts of both animals. The GPTs (64 h) for Co associated with AVS particles in M. edulis were an order of magnitude greater than for Ag and Cd, or for Co associated with other particle types. Overall, no single mechanism appears to control metal AE in marine bivalves and in vitro studies of metal dissolution in gut juice do not completely mimic the complex digestive processes operating in vivo, and thus cannot fully explain metal assimilation in these animals.