Interstitial waters from eleven sites of the Eastern Mediterranean Basin (Sites 963–973) were analyzed for stable isotopes of dissolved sulfate (δ 34 S, δ 18 O) and major and minor ions. Sulfate reduction rates are positively related to bulk sedimentation rates, which indicates a higher burial of metabolizable organic matter with increasing sedimentation rates. Bacterial sulfate reduction in the deeper samples from most of the sites is superimposed by a sulfate input from Messinian evaporites or late-stage evaporite brines that are located at depth; dissolution of gypsum within the cored section was found at Sites 967 and 968. Authigenic gypsum precipitation was identified at Site 973 below 100 mbsf. Intense sulfate reduction is also indicated for the pore waters from the mud volcanoes of Sites 970 and 971. In addition to high concentrations of hydrocarbons (mainly CH 4), coexisting H 2 S and SO 4 2– were also present, indicating that methane perco-lates through the sediment from greater depths. The observed variabilities in sulfate concentrations between different holes of Sites 970 and 971 are caused mainly by locally varying-upward fluxes of methane. Extremely high alkalinity values in the pore waters of Sites 970 and 971 are the result of microbial CH 4 oxidation. The concentration and sulfur isotopic composition of pore-water sulfate (δ 34 S values up to +112 ‰ vs. the Vienna-Canyon Diablo troilite standard) are dominated by microbial organic matter degradation with associated sulfate reduction. Therefore, most interstitial fluids are enriched in 34 S with respect to modern Mediterranean seawater (δ 34 S = +20.7‰; Site 973 surface seawater). δ 18 O(SO 4 2–) values at Site 963 and 964 are also enriched in 18 O with respect to Mediterranean seawater (δ 18 O[SO 4 2– ] = +9.4‰). δ 34 S and δ 18 O values of dissolved residual sulfate are positively related to each other. An initial kinetic isotope effect is superimposed by oxygen isotope exchange reactions leading to an increased equilibration between residual sulfate and pore water with increasing degree of sulfate reduction. It is suggested that δ 18 O−δ 34 S relations of residual sulfate directly reflect sulfate reduction rates in marine sediments.