We present a new fiber-optic oxygen microsensor based on dynamic luminescence quenching which was recently developed for measuring oxygen at high spatial resolution in aquatic sediments and biofilms. Micro-optrodes with a typical tip diameter of 20 to 50 micrometers were fabricated. The fabrication procedure is simple and guarantees a high reproducibility of the calibration curves. The micro-optrodes were characterized with respect to dynamic range, response time, storage, longterm stability, interferences, temperature dependence, photostability, and mechanical stability. A special LED based luminescence intensity measuring instrument was developed. It is battery operated and can be used for field measurements. The micro-optrodes were used to measure oxygen gradients in marine sediments. Comparative measurements were performed with oxgen microelectrodes. The first measurements have shown that oxygen micro-optrodes present a true alternative to existing electrochemical microsensors. Nevertheless, it is obvious, that the measurement of luminescence intensity of the indicator limits their practical application. Therefore a new setup was developed to make oxygen measurements with the luminescence lifetime as parameter.