1 ) A method of determining the composition of a multiphase fluid comprising at least two mixed phases, comprising: a) selecting a mixture law that expresses a mean permittivity of the fluid as a function of permittivity and volume fraction of each phase; b) measuring an attenuation and a phase shift of at least two microwave beams of different frequencies that have traversed test fluids of known composition; c) from the mixture law and from the attenuation and phase shift measurements performed in b), determining for each one of the frequencies a measurement model that expresses the attenuation and the phase shift of a microwave beam as a function of the volume fraction of each phase; d) for each one of the frequencies, measuring an attenuation and a phase shift pair for at least one microwave beam that has traversed said multiphase fluid; e) determining the composition of the multiphase fluid by determining a value of volume fractions of the phases that minimize a function taking accounting for the distances, calculated for each one of the frequencies, between a point whose coordinates correspond to an attenuation and the phase shift pair measured for a value of the frequencies and a point whose coordinates correspond to the attenuation and phase shift pair determined by one of the models valid for the same frequency value.