The effect of oxygen on the degradation of sedimentary organic matter has been determined for 6 subtidal stations and 3 intertidal stations in the North Sea area. The stations were selected to cover a range of organic matter lability and sediment texture (and hence concentrations of organic matter). Slurry incubations revealed that at low mineralisation rates, aerobic mineralisation is significantly faster than anaerobic mineralisation, irrespective of the degree of lability of organic matter. A complementary incubation experiment with sediment rich in organic carbon mixed with varying proportions of organically poor sediments confirmed the enhanced aerobic mineralisation at low mineralisation levels. It is proposed that oxygen-enhanced degradation occurs at low mineralisation levels at which bacterial biomass production becomes limiting. [KEYWORDS: organic matter; degradation; oxygen; sediments; North Sea; intertidal Long-island sound; continental-margin sediments; green algal cells; marine-sediments; carbon preservation; sulfate reduction; amino-acids; anaerobic mineralization; microbial decomposition; chlorophyll-a] ; The effect of oxygen on the degradation of sedimentary organic matter has been determined for 6 subtidal stations and 3 intertidal stations in the North Sea area. The stations were selected to cover a range of organic matter lability and sediment texture (and hence concentrations of organic matter). Slurry incubations revealed that at low mineralisation rates, aerobic mineralisation is significantly faster than anaerobic mineralisation, irrespective of the degree of lability of organic matter. A complementary incubation experiment with sediment rich in organic carbon mixed with varying proportions of organically poor sediments confirmed the enhanced aerobic mineralisation at low mineralisation levels. It is proposed that oxygen-enhanced degradation occurs at low mineralisation levels at which bacterial biomass production becomes limiting. [KEYWORDS: organic matter; degradation; oxygen; sediments; North Sea; intertidal Long-island sound; continental-margin sediments; green algal cells; marine-sediments; carbon preservation; sulfate reduction; amino-acids; anaerobic mineralization; microbial decomposition; chlorophyll-a]