A fuel-gauging system has a capacitive sensor that is mounted in a fuel tank and supplied with an alternating electric signal from an oscillator. The oscillator output is supplied to a diode-pump circuit that produces a D.C. voltage proportional to the product of the amplitude and the frequency of the oscillator output. The D.C. voltage is supplied to the input of a comparator that receives a constant voltage at its other input. The comparator produces a D.C. output voltage, dependent on the difference between its inputs, which is supplied to an input of the oscillator, the oscillator being arranged such that the amplitude of its output voltage is dependent upon the amplitude of the input voltage. In this way, the product of the amplitude and frequency of the oscillator output is maintained substantially constant. The system may have another capacitor located in the fuel tank to enable compensation for temperature changes, and a further capacitor of open construction immersed in the fuel, arranged to enable compensation for changes in permittivity of the fuel.