Surface sediments from the north-eastern coast of the Marmara Sea, Turkey's most industrialized coastal region, were enriched with radioisotopes from the Chernobyl explosion in 1986. Caesium-137 in these sediments is also thought to originate from one former paper mill located nearby that used wood contaminated by Chernobyl explosion-originated Cs-137 for paper production. The average activity concentration of the Cs-137 was 21 Bq kg(-1), while naturally occurring radioisotopes, i.e. K-40, Ra-226, and Ra-228, were 568, 18 and 24 Bq kg(-1), respectively, in surface sediments. The natural radionuclide activities reached their highest levels near petrochemical, phosphate and fertilizer processing facilities. Average Cs-137 activities were generally up to ten times higher than in Middle Eastern marine sediments and lower than those in Northern European sediments. (C) 2013 Elsevier Ltd. All rights reserved.