Abstract: Methods and apparatus for determining the relative proportions of intermixed constituents of a multiphase fluid system. The constituents for which relative proportions are being measured must have measurably different electrical permittivities. The systems use at least one electrically conductive probe having two conductors which are spaced apart and capable of receiving the fluid system therebetween. Time domain reflectometers present a stimulating time varying electrical signal to the probes and receive and measure reflected signals generated at impedance changes in the transmission line and probe. The probe transit time and/or associated average probe velocity provide a measure of the fluid system conditions which can be compared to predetermined values corresponding to various relative proportions of the intermixed constituents. A measure of the relative portions of the constituents is thus provided.