Estuarine sediments from upper New Bedford Harbor, Massachusetts, which were heavily contaminated with polychlorinated biphenyls (PCBs), showed considerable compositional alterations of PCB residues relative to mixtures of Aroclor 1242 and 1254 standards, and PCB distributions present in sediments from lower New Bedford Harbor and Black Rock Harbor, Connecticut. The compositional alterations, which presumably resulted from reductive dechlorinations by bacteria, became more extensive with increasing PCB concentration and decreasing distance from the suspected PCB source, an electrical capacitor manufacturing plant. The dechlorination processes found may decrease the potential toxicity and bioaccumulation potential of the PCB residues in these sediments.