Mineralization of a readily biodegradable aromatic compound (benzoate) by intrinsic microorganisms in the anoxic sediment was used to quantify the inhibitory effect of heavily contaminated sediment from the Arthur Kill estuary (NY/NJ Harbor system, USA) on the anaerobic metabolism by naturally present bacterial populations. In anoxic microcosms, the effect of varying ratios of contaminated sediment: site water and contaminated sediment: noncontaminated sediment (Flax Pond, Stony Brook, NY, USA) were investigated. In all cases, increasing the ratio of Arthur Kill sediment in the microcosms showed an inhibitory effect on the rate of 14C-benzoate mineralization as measured by the evolution of 14CO2. This inhibitory effect could be alleviated through dilution of the sediment with noncontaminated sediment, resulting in some cases in mineralization rates that were greater by an order of magnitude. The toxicity of the sediment was confirmed by whole-sediment Microtox® bioassay. Analysis of the sediment revealed high (>200 mg/kg) levels of Pb, Cu, Zn, and Cr, suggesting that heavy metals may contribute to overall sediment toxicity.