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Mist streams comprised of H2O2/H2O droplets are a recent innovation for disinfecting medical equipment, but the availability of a sensor that could monitor the concentration of H2O2 applied during the treatment would be desirable. Here we describe a means to obtain a rapid estimation of H2O2 concentration in this environment. The proposed sensor is based on a platinum resistance thermometer coated with a layer of MnO2 catalyst. It may be calibrated to operate either during the mist delivery step of a disinfection cycle, or during the evacuation (drying) phase. Cooling of the sensor surface due to evaporation of H2O and effervescence of decomposing H2O2 operates against heat generated by the decomposition reaction to produce a well-defined minimum in the temperature. The time and temperature at which this minimum occurs are well correlated, with the H2O2 content of the solution used to produce the mist droplets.