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Black platinum-modified ultramicroelectrodes (UME) were previously reported as excellent sensors for hydrogen peroxide produced by biological systems; their detection limit being typically 100 nM. In the present work, we took benefit of an ultramicroelectrode array configuration (16×20 μm diameter-disk UME) to increase the overall electroactive surface and consequently, faradaic currents. Platinum-UME arrays integrated on a silicon chip were characterized by cyclic voltammetry before and after platinization for ferricyanide reduction and hydrogen peroxide oxidation in comparison with a single platinized UME. Then, UME arrays were platinized at different coulometric charges (0.04 μC.μm−2 to 3.2 μC.μm−2) to determine the best compromise between faradaic and capacitive currents associated with the hydrogen peroxide oxidation wave. Sensitivity and detection limit for H2O2 of these different platinized UME arrays were studied by chrono-amperometry at +300 mV vs Ag/AgCl. The detection limit was lowered to 10 nM H2O2 in buffer with the UME arrays, which is an excellent condition for biological measurements. Black platinum-modified UME arrays were eventually used to monitor the production of H2O2 by mitochondria under a condition favoring the oxidative stress pathway; a flux of few nanomoles H2O2/mg protein/min. was detected with kinetic and quantitative accuracy.