Many regulatory frameworks for sediment quality assessment include consideration of contaminant bioavailability. However, the “snap-shots” of metal bioavailability provided by analyses of porewaters or acid-volatile sulfidesimultaneously extractable metal (AVS-SEM) relationships do not always contribute sufficient information. The use of inappropriate or inadequate information for assessing metal bioavailability in sediments may result in incorrect assessment decisions. The technique of diffusive gradients in thin films (DGT) enables the in situ measurement of metal concentrations in waters and fluxes from sediment porewaters. We utilized the DGT technique to interpret the bioavailability of copper to the benthic bivalve Tellina deltoidalis in sediments of varying properties contaminated with copper-based antifouling paint particles. For a concentration series of copper-paint contaminated sandy, silty-sand, and silty sediment types, DGTprobes were used to measure copper fluxes to the overlying water, at the sedimentwater interface, and in deeper sediments. The overlying water copper concentrations and DGT-Cu fluxes were shown to provide excellent exposure concentration−response relationships in relation to lethal effects occurring to the copper-sensitive benthic bivalve, T. deltoidalis. The study demonstrates the strength of the DGT technique, which we expect will become frequently used for assessing metal bioavailability in sediments.