The adsorption-desorption behavior and bioavailability of mercury and cadmium in a model salt marsh ecosystem have been investigated. Specimens of two species of filter-feeding mussels, Mytilus edulis and Modiolus demissus, were placed in large rectangular enclosures containing marsh peat previously equilibrated with a seawater solution of the metals. The tissue concentration was determined after 21 and 70 days. Correlation between the levels of these metals in mussel tissue and the following parameters were examined: percent total organic matter, sediment metal concentration, acid leachable metal concentration, and amounts of metal desorbed from the sediment. Multiple regression analysis indicated a strong dependence of sorption-desorption processes on sediment organic matter content for both metals. Uptake of mercury was found to depend primarily on the amounts of acid leachable metal and the amounts of metal desorbed from the sediment. Cadmium uptake depended primarily on percent total sedimentary organic matter and the amounts of metal desorbed.