Detailed analyses of total sulfur, pyritic sulfur, humic sulfur, NaCl-extractable sulfur, elemental sulfur, acid-volatile sulfide, and organic polysulfide in and around the most recent Sapropel S1 (maximum C org = 2.3%) and two other sapropels recovered during Ocean Drilling Program Leg 160 (maximum C org = 7.4% and 23.5%), show that the main sulfur species in and immedi-ately below each sapropel is pyrite. Directly above each sapropel sulfur is rarely present in the solid phase, but occurs as pore-water sulfate (SO 4 2–). Microbial SO 4 2– reduction took place in the sapropels during sapropel formation. Addition of reactive iron to sapropel layers occurred via upward diffusion of Fe 2+ from underlying sediments and/or through water-column iron sul-fide precipitation. All reactive iron that was available in the sediments was used for pyrite formation. As a result, diffusion of sulfide out of the sapropels and sulfidization of the sediments underlying each sapropel have occurred. Only in the most C org--rich sapropel did large-scale uptake of reduced sulfur by organic molecules occur, and SO 4 2– reduction probably still continues.