The release of four individual polychlorinated biphenyls (2,3',5-trichlorobiphenyl, 2,2',4,5'-tetrachlrobiphenyl, 2,2',4,5,5'- and 2,2',3',4,5-pentachlorobiphenyl) from heavily contaminated Waukegan Harbor, Illinois, USA, sediments was studied in a laboratory microcosm. Polychlorinated biphenyls released from the sediments to overlying water were collected on a polyurethane foam trap. Release rate was a function of sediment concentration, chlorine substitution pattern and degree of chlorination. Because of their small apparent diffusivity, PCBs in contaminated sediments are quickly removed from communication with overlying water if the sediments are not affected by physical resuspension, bioturbation or bioaccumulation.