Gas hydrates and gas bubbles were collected during the MARNAUT cruise (May-June 2007) in the Sea of Marmara along the North Anatolian Fault system, Turkey. Gas hydrates were sampled in the western part of the Sea of Marmara (on the Western High), and three gas-bubble samples were recovered on the Western High, the Central High (center part of the Sea of Marmara) and in the Cinarcik Basin (eastern part of the Sea of Marmara). Methane is the major component of hydrates (66.1%), but heavier gases such as C-2, C-3, and i-C-4 are also present in relatively high concentration. The methane contained within gas hydrate is clearly thermogenic as evidenced by a low C-1/C-2 + C-3 ratio of 3.3, and carbon and hydrogen isotopic data (delta C-13(CH4) of -44.1 parts per thousand PDB and delta D-CH4 of -219 parts per thousand SMOW). A similar signature is found for the associated gas bubbles (C-1/C-2 + C-3 ratio of 24.4, delta C-13(CH4) of -44.4 parts per thousand PDB) which have the same composition as natural gas from K. Marmara-af field. Gas bubbles from Central High show also a thermogenic origin as evidenced by a C-1/C-2 + C-3 ratio of 137, and carbon and hydrogen isotopic data (delta C-13(CH4) of -44.4 parts per thousand PDB and delta D-CH4 of -210 parts per thousand SMOW), whereas those from the Cinarcik Basin have a primarily microbial origin (C-1/C-2 + C-3 ratio of 16,600, delta C-13(CH4) of -64.1 parts per thousand PDB). UV-Raman spectroscopy reveals structure II for gas hydrates, with CH4 trapped in the small (5(12)) and large (5(12)6(4)) cages, and with C2H6, C3H8 and i-C4H10 trapped in the large cages. Hydrate composition is in good agreement with equilibrium calculations, which confirm the genetic link between the gas hydrate and gas bubbles at Western High and the K.Marmara-af offshore gas field located north of the Western High. We calculate the characteristics of the hydrate stability zone at Western High and in the Cinarcik Basin using the CSM-GEM computer program. The base of the structure II hydrate stability field is at about 100 m depth below the seafloor at the Western High site, whereas in the Cinarcik Basin, P-Tconditions at the sea floor correspond to the uppermost range for structure I hydrate formation from microbial gas. Published by Elsevier B.V.