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A new fiber‐optic oxygen microsensor (microoptrode) based on dynamic fluorescence quenching has been developed to measure oxygen gradients in marine sediments and microbial mats. The microoptrodes are fabricated by immobilizing an oxygen‐quenchable fluorophore at the tapered tip of an optical fiber. A special optoelectronic system has been designed to measure oxygen with these microoptrodes. It is based on small and cheap optical components and can easily be miniaturized for field applications. In contrast to oxygen microelectrodes, the new oxygen microoptrodes are easy to make, do not consume oxygen, and show no stirring dependence of the signal. In addition, they show excellent long‐term stability and storage stability. Hydrogen sulfide, carbon dioxide, and other relevant chemical parameters do not interfere with the measurement. Oxygen profiles in marine sediments obtained from …