The samples presented here were collected with the intent to understand methane cycling in the sediments of the Baltic Sea as part of the BALTIC GAS project funded by the European Community's Seventh Framework Programme (FP/2007-2013) under grant agreement n° 217246 made with the joint Baltic Sea Research and Development programme BONUS. Sediment cores from the Baltic Sea were collected and serve as events in this archive. The events took place in Mecklenburg Bay, Arkona Basin, Bornholm Basin, Gotland Basin, Bothnian Sea, and Bothnian Bay. Ninety events took place between July 31- August 15, 2010 aboard RV Maria S. Merian cruise MSM16/1. From the sediment cores, porewater was extracted and analyzed for sulfate, chloride, ammonium, major ions (barium, calcium, potassium, magnesium, sodium), hydrogen sulfide, and dissolved inorganic carbon concentrations. Dissolved inorganic carbon was also measured for stable carbon isotope ratios. Sediment plugs were collected for measuring concentrations and stable carbon isotope ratios of methane. Sulfate reduction rates were also measured on sediment cores. Bulk sediment was then analyzed for total organic carbon and nitrogen content, along with stable carbon and nitrogen ratios on total organic matter. Sediments were also measured for wet bulk density and water content of wet mass, from which porosity was calculated.