Summary: Benthic total oxygen uptake (TOU) was measured in two Arctic fjords (NW Spitsbergen shelf) with different hydrological regimes: Hornsund with “cold” coastal Arctic waters and Kongsfjorden with “warm” Atlantic shelf waters. TOU rates in Kongsfjorden were more than 50% higher than in Hornsund. This is presumably related to the relatively higher biomass of bacterial and faunal (meiobenthos and macrofauna) communities in Kongsfjorden as compared to Hornsund caused by the source of organic matter: Kongsfjorden is dominated by marine, Hornsund by terrigenous organic matter. We conclude that the quality of organic matter supplied to marine sediments influences the biomass of benthic organisms and the rate of oxygen consumption. Therefore the Kongsfjorden sea bed has much higher oxygen uptake and hence a greater carbon demand than Hornsund. Keywords: Sediment oxygen uptake, Respiration partitioning, Carbon demand, Svalbard fjord, Arctic