The relative humidity of supercritical carbon dioxide, under realistic process conditions, has been measured using an extraction plant. A wet-chemical method for the precise measurement, of humidity was optimized for calibrations. A capacitive humidity sensor consisting of an Al2O3 ceramic capacitor was then tested for its suitability in supercritical fluids, Based on experimental results, a physical-based phenomenological model was developed to describe the dependence of the sensor signal on the humidity, pressure, and temperature, The model facilitates calibration of the sensor, The model is applicable to other systems and is not restricted to the supercritical state, The capacitive measurement proved to be useful up to a relative humidity value of Phi = 0.7 in the temperature range from 31 to 75 degrees C at pressures between 100 bar and 350 bar.