Pyrite formation within and directly below sapropels in the eastern Mediterranean was governed by the relative rates of sulphide production and Fe liberation and supply to the organic-rich layers. At times of relatively high SO42- reduction, sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments. The sources of Fe for pyrite formation comprised detrital Fe and diagenetically liberated Fe(ll) from sapropel-underlying sediments. In organic-rich sapropels, input of Fe from the water column via Fe sulphide formation in the water may have been important as well. Rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels, whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred. delta(34)S values of pyrite are -33 parts per thousand to -50 parts per thousand. Below the sapropels delta(34)S is lower than within the sapropels, as a result of increased sulphide re-oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment. The percentage of initially formed sulphide that was re-oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment. It ranges from 34% to 80%, varying significantly between sapropels. Increased palaeoproductivity as well. as enhanced preservation contributed to magnified accumulation of organic matter in sapropels. [KEYWORDS: sapropel; pyrite; framboidal microtexture; S-34/S-32; organic carbon; eastern Mediterranean Organic-carbon preservation; marine-sediments; postdepositional oxidation; laboratory synthesis; sulfur isotopes; productivity; sea; matter; diagenesis; evolution] ; Pyrite formation within and directly below sapropels in the eastern Mediterranean was governed by the relative rates of sulphide production and Fe liberation and supply to the organic-rich layers. At times of relatively high SO42- reduction, sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments. The sources of Fe for pyrite formation comprised detrital Fe and diagenetically liberated Fe(ll) from sapropel-underlying sediments. In organic-rich sapropels, input of Fe from the water column via Fe sulphide formation in the water may have been important as well. Rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels, whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred. delta(34)S values of pyrite are -33 parts per thousand to -50 parts per thousand. Below the sapropels delta(34)S is lower than within the sapropels, as a result of increased sulphide re-oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment. The percentage of initially formed sulphide that was re-oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment. It ranges from 34% to 80%, varying significantly between sapropels. Increased palaeoproductivity as well. as enhanced preservation contributed to magnified accumulation of organic matter in sapropels. [KEYWORDS: sapropel; pyrite; framboidal microtexture; S-34/S-32; organic carbon; eastern Mediterranean Organic-carbon preservation; marine-sediments; postdepositional oxidation; laboratory synthesis; sulfur isotopes; productivity; sea; matter; diagenesis; evolution]