Multi-proxy analyses and lithology of two cores, MRS-CS18 and MRS-CS27, from the Imrali Basin of the Sea of Marmara (SoM) provide novel information on environmental conditions, relative sea level, and sill depths of the straits of Bosporus and Dardanelles during the Marine Isotope Stages (MIS) 5 and 6. The fossil and multi-proxy geochemical records show that lacustrine conditions prevailed in the SoM during most of MIS 6, from 171 to 134 ka BP, and that the transition to marine conditions during Termination II took place at similar to 134.06 +/- 1.10 ka BP. MIS 5 interstadials a, c, and e witnessed the formation of three sapropels (MSAP-2, MSAP-3 and MSAP-4) under suboxic to anoxic marine conditions, whereas during stadials MIS 5b (similar to 94-86) and MIS 5d (similar to 112-105 ka BP), lacustrine and marine conditions with deposition of sediments having relatively low TOC contents (<2%) prevailed, respectively. Consideration of the global sea level, together with the timing of the marine reconnection of the SoM during Termination II and persistence of the marine conditions during MIS 5, except for MIS 5b and later part of MIS 5a, suggests that the Dardanelles sill depth was at similar to -75 +/- 5 m during the reconnection at Termination II and at-55 +/- 5 m during most of MIS 5. On similar considerations of the Black Sea marine reconnections and disruptions during the MIS 5, a sill depth of -35 to-40 m (similar to the present day depth) is indicated for the Bosporus Strait.