Sediment oxygenation was assessed in situ in rhizospheres of the intertidal salt marsh grass, Spartina anglica. Oxygen content of the rhizosphere was investigated in two populations of S. anglica with differing plant morphology growing in different sediment types. The oxygen content was assessed in situ using a novel multifiber optode system with 100 simultaneously operated oxygen probes. No oxygen was detected inside the rhizosphere at any depths indicating a limited impact of plant-mediated sediment oxygenation on the bulk anoxic sediment. Additional Planer optode investigations revealed that plant-mediated sediment oxygenation do occur, but it is restricted to small areas around the root-tips.