Abstract: An apparatus and method for determining the phase volume fractions for a multiphase oil well effluent is disclosed. The apparatus measures multiple energy gamma ray attenuations through the effluent mixture, and the method determines the phase volume fractions from these measurements when the mixture is under inhomogeneous flow conditions. The apparatus includes a pipe section with a Venturi through which the mixture flows. A source of radiation transmits rays at two or more energy levels through the mixture. A solid state detector measures the transmitted attenuation count rates. A low absorption window of a truncated cone shape is preferably incorporated in lieu of the pipe wall in the area of the detector. The method comprises measuring ray attenuation, in the two-energy example, at one high energy emission line, to differentiate between the gas and liquid components, and at a relatively lower energy emission line, to differentiate between the oil and water phases within the liquid component.