A method of determining the void rate in a biphase gas/liquid medium, corresponding to the volume fraction of gas corresponding to the presence of bubbles in the liquid medium in a total volume of gas and liquid, comprises: deployment of a bulk elastic wave resonator in contact and coupled acoustically with the biphase medium; measurement by nonlinear resonant ultrasound spectroscopy of the biphase medium comprising the scanning in terms of frequencies and amplitudes in a given range of frequencies and in a given range of amplitudes, of bulk elastic waves emitted and detected at said resonator placed in said medium and leading a set of resonance curves exhibiting maxima; determination of a straight line defined by the set of maxima of said curves and of the slope of said straight line; determination of the void rate on the basis of said slope. The method may be applied to a nuclear reactor.