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We report here a novel approach for rapid sensing of oxygen in solution. Dissolved oxygen is measured using an ultramicroelectrode array (UMEA) (36 UMEs; 2μm Ø 20μm spacing) fabricated from Pt in an oxide-nitride-oxide insulating layer on glass. The oxygen concentration is determined in the short time measurement mode using the slope of the measured current I as a function of 1/√t (-0.4V applied), t being the measurement time (Cottrell equation). Specifically, the dissolved oxygen concentration is measured within less than 10ms in the solution. At the same time, the concentration in dissolved oxygen is monitored using an external optical sensor, for calibration purposes. A linear relationship (R2=0.96) is found between the responses of the two sensors. By implementing a conditioning step (0.1V applied for 10ms), only a 2% change is observed in the UMEA response over a 2h period. The proposed sensing approach will be very valuable to monitor in situ the respiratory activity of microtissues in nL volumes, as the amount of oxygen consumed by the sensor is dramatically decreased using short measurement times.