A probe based on the measurement of the electrical impedance has been developed, to determine the phase hold-ups in gas-solid-liquid stirred tank reactors. The idea was to take advantage of the complex nature of electrical impedance to derive two,,independent, data: (conductance and capacitance) from. one measurement Particular attention has been paid to the calibration of the probe and two semi-empirical equations have been given to relate measurement to hold-ups. Subsequently; experiments were performed on three-phase and two-phase (gas-liquid and solid-liquid), systems and the results have been critically analysed.