After the Marmara (Izmit) Earthquake (magnitude 7.4) on 17 August 1999, chemical oceanographical characteristics of Izmit Bay were investigated in order to examine the possible effects of the refinery fire and uncontrolled entrance of domestic wastes to the surface waters. The dissolved oxygen (DO) content of the water column in August 1999 was the lowest value of all the measurement periods. It was found to be lower than the detection limit of the method (0.03 mg l(-1)) in the lower layer of eastern and central basins of the Bay. whereas the dissolved hydrogen sulfide (DHS) values were high, varying between 0.14 and 1.28 mg l(-1). The deficiency of DO and in turn formation of DHS were caused by the spreading petroleum from the refinery fire onto the sea surface and waste loads from the damaged municipal waste effluent system. The increasing organic/inorganic loads into the Bay stimulated the phytoplankton blooms which cause locally saturated DO concentrations in the eastern basin during autumn 1999. DO concentrations increased in lower layer waters during winter, whilst DHS formation disappeared when water originating from the Marmara Sea replenish the water column of Izmit Bay. However, DHS formation established again in August 2000. (C) 2003 Elsevier Science Ltd. All rights reserved.