A method and device includes measuring a multiple-phase fluid flowing through a pipe. The inventive method includes the following steps in which: a fluid is illuminated using a first coaxial probe placed in contact with the liquid phase of the fluid, and a first electromagnetic wave is emitted at a high frequency. The admittance at the interface between the first probe and the fluid is measured, and the fractions of at least two constituents of the liquid phase are calculated in order to obtain the effective permittivity (∈1) of the phase. The method also includes the following steps in which: the fluid is illuminated using a second coaxial probe and a second electromagnetic wave is emitted at a low frequency. The admittance at the interface between the second probe and the liquid is measured, and the thickness (e) of the liquid is measured. The aforementioned calculation is performed on the basis of the calculated effective permittivity (∈1) and the admittance measured by the second probe.