Dechlorination kinetics of polychlorinated biphenyls (PCBs) were investigated in Aroclor 1248-spiked sediments at 16 concentrations ranging from 0 to 200 ppm using sediment microorganisms from the Reynolds site in the St. Lawrence River, New York, USA, over a 58-week incubation period. The time course of dechlorination, measured as the total Cl per biphenyl, consisted of an initial lag phase followed by rapid dechlorination and then a plateau that represented an apparent endpoint of dechlorination. A clear threshold concentration was found between 35 and 45 ppm; there was no dechlorination observed at seven concentrations below this level. Above the threshold concentration, dechlorination rate was a function of sediment PCB concentration. The rate, calculated as the slope of the rapid phase, was linear within the concentration range investigated. The maximum extent of dechlorination also increased with initial Aroclor concentrations; only 4% of Cl per biphenyl was removed at 45 ppm, and the removal was saturated at approximately 36% above 125 ppm. This difference appeared to be due to whether or not dechlorination involved meta-rich congeners such as . These results indicate that a major controlling factor for natural remediation potential in sediments is the initial PCB concentration that determines the maximum extent of dechlorination rather than the dechlorination rate.