Abstract The 15 N/ 14 N isotopic ratio of marine particulate organic matter usually increases as a consequence of preferential 14 N-remineralization (ammonification) during decay in the oxic water-column and at the oxic seafloor. However, a fractionation factor 15 e of sediment ammonification in natural environments has not been determined yet. Here, the 15 e of sedimentary ammonification is calculated in a set of seven sapropel containing sediment cores, two subrecent sediment cores and four sediment trap deployments from the Eastern Mediterranean Sea. Data clearly reveal Rayleigh type isotope fractionation and a fractionation factor 15 e between −1.43‰ and −2.3‰. Hence, sediments – and probably sinking particulate organic matter as well – can be important sources of isotopically depleted reactive nitrogen. The knowledge of 15 e further enables to reconstruct paleo export fluxes from sediment cores if original and actual δ 15 N as well as N accumulation rates are known.