Dissolved organic matter (DOM) in marine sediments is a complex mixture of thousands of individual constituents which are involved in various biogeochemical reactions and serve as substrates for benthic microbes. The bioavailability of DOM compounds is controlled by their size and reactivity. Knowledge of the molecular composition of DOM is a prerequisite for a comprehensive understanding of the biogeochemical processes in sediments. In this study, interstitial water DOM was extracted with Rhizon samplers from a sediment core from the Black Sea and compared to the water-extractable organic matter fraction (