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This paper introduces a new ultraminiature, ultrahigh sensitivity moisture sensor that is based on ultracapacitor technology. An ultracapacitor is assembled from ordinary activated carbon electrodes, however, unlike ordinary ultracapacitors, no liquid electrolyte exists inside the device. The ultracapacitor electrodes are placed on both sides of a layer of porous silicon in which potassium hydroxide in powder form is embedded. As moisture penetrates the porous silicon layer, a liquid electrolyte starts to form. The conductivity of the electrolyte, and hence the capacitance of the ultracapacitor assembly, increase as the amount of moisture increases. In the present prototype, an increase in the relative humidity from 5% to 80% results in a capacitance variation from 0 to 17 μF. The sensor can be very useful for monitoring moisture penetration inside small electronic devices and packages that are sensitive to moisture.