The paper presents a mass balance modeling study on the polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) pollution around the Izmit Bay region, the most industrialized province of Turkey, based on the evaluation of PCDD/Fs input to the bay and measured concentrations in the surface sediments. The pollution map showed that PCDD/F levels in the central section are much higher than those in the eastern and western basins. The model showed that OCDD levels could be explained mainly by the effect of atmospheric deposition processes whereas other dominant congeners i.e., Octachlorodibenzofuran (OCDF) and 1,2,3,4,6,7,8-Heptachlorodibenzofuran (1,2,3,4,6,7,8-HpCDF), observed in the surface sediment, should have another unknown sources. Assessment of the congener profiles of the industrial discharges in the recent studies showed that OCDF was related to the production of vinyl chloride monomer in the past, 1,2,3,4,6,7,8-HpCDF was attributed to unknown use of the chlorophenol-based fungicide. On the other hand, atmospheric OCDD could be explained by the effect of combustion-related PCDD/F emissions, and the use of pentachlorophenol (PCP) pesticide. In conclusion, the mass balance model results suggested that the industrial discharges in the past could be a significant contributor to the PCDD/F pollution in the sediments. (C) 2020 Elsevier B.V. All rights reserved.